## What changes were proposed in this pull request?

Made DataFrame-based API primary
* Spark doc menu bar and other places now link to ml-guide.html, not mllib-guide.html
* mllib-guide.html keeps RDD-specific list of features, with a link at the top redirecting people to ml-guide.html
* ml-guide.html includes a "maintenance mode" announcement about the RDD-based API
  * **Reviewers: please check this carefully**
* (minor) Titles for DF API no longer include "- spark.ml" suffix.  Titles for RDD API have "- RDD-based API" suffix
* Moved migration guide to ml-guide from mllib-guide
  * Also moved past guides from mllib-migration-guides to ml-migration-guides, with a redirect link on mllib-migration-guides
  * **Reviewers**: I did not change any of the content of the migration guides.

Reorganized DataFrame-based guide:
* ml-guide.html mimics the old mllib-guide.html page in terms of content: overview, migration guide, etc.
* Moved Pipeline description into ml-pipeline.html and moved tuning into ml-tuning.html
  * **Reviewers**: I did not change the content of these guides, except some intro text.
* Sidebar remains the same, but with pipeline and tuning sections added

Other:
* ml-classification-regression.html: Moved text about linear methods to new section in page

## How was this patch tested?

Generated docs locally

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #14213 from jkbradley/ml-guide-2.0.

## What changes were proposed in this pull request?

This PR tries to fix all typos in all markdown files under `docs` module,
and fixes similar typos in other comments, too.

## How was the this patch tested?

manual tests.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11300 from dongjoon-hyun/minor_fix_typos.

[SPARK-12212][ML][DOC] Clarifies the difference between spark.ml, spark.mllib and mllib in the documentation.

Replaces a number of occurences of `MLlib` in the documentation that were meant to refer to the `spark.mllib` package instead. It should clarify for new users the difference between `spark.mllib` (the package) and MLlib (the umbrella project for ML in spark).

It also removes some files that I forgot to delete with #10207

Author: Timothy Hunter <timhunter@databricks.com>

Closes #10234 from thunterdb/12212.

Author: Pravin Gadakh <pravingadakh177@gmail.com>
Author: Pravin Gadakh <prgadakh@in.ibm.com>

Closes #9340 from pravingadakh/SPARK-11380.

In the Markdown docs for the spark.mllib Programming Guide, we have code examples with codetabs for each language. We should link to each language's API docs within the corresponding codetab, but we are inconsistent about this. For an example of what we want to do, see the "ChiSqSelector" section in https://github.com/apache/spark/blob/64743870f23bffb8d96dcc8a0181c1452782a151/docs/mllib-feature-extraction.md
This JIRA is just for spark.mllib, not spark.ml.

Please let me know if more work is needed, thanks a lot.

Author: Xin Ren <iamshrek@126.com>

Closes #8977 from keypointt/SPARK-10669.

1, Add Python example for mllib FP-growth user guide.
2, Correct mistakes of Scala and Java examples.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #8279 from yanboliang/spark-10084.

Updates FPM user guide to include Association Rules.

Author: Feynman Liang <fliang@databricks.com>

Closes #8207 from feynmanliang/SPARK-9900-arules.

Adds user guide for `PrefixSpan`, including Scala and Java example code.

mengxr zhangjiajin

Author: Feynman Liang <fliang@databricks.com>

Closes #8253 from feynmanliang/SPARK-9898.

There is a mistake in the URLs of the Scala section of FP-Growth in the MLlib Frequent Pattern Mining documentation. The URL points to https://spark.apache.org/docs/latest/api/java/org/apache/spark/mllib/fpm/FPGrowth.html which is the Java's API, the link should point to the Scala API https://spark.apache.org/docs/latest/api/scala/index.html#org.apache.spark.mllib.fpm.FPGrowth

There's another mistake in the FP-GrowthModel in the same section, the link points, again, to the Java's API https://spark.apache.org/docs/latest/api/java/org/apache/spark/mllib/fpm/FPGrowthModel.html, the link should point to the Scala API https://spark.apache.org/docs/latest/api/scala/index.html#org.apache.spark.mllib.fpm.FPGrowthModel

Author: FavioVazquez <favio.vazquezp@gmail.com>

Closes #6722 from FavioVazquez/fix-wrog-urls-mllib-fpgrowth and squashes the following commits:

e1ca54d [FavioVazquez] - Fixed wrong URLs in MLlib Frequent Pattern Mining, FP-Growth Scala section
ad882a3 [FavioVazquez] Merge remote-tracking branch 'upstream/master'
f27a20b [FavioVazquez] Merge remote-tracking branch 'upstream/master'
9af7074 [FavioVazquez] Merge remote-tracking branch 'upstream/master'
edab1ef [FavioVazquez] Merge remote-tracking branch 'upstream/master'
b2e2f8c [FavioVazquez] Merge remote-tracking branch 'upstream/master'

In the previous version, PIC stores clustering assignments as an `RDD[(Long, Int)]`. This is mapped to `RDD<Tuple2<Object, Object>>` in Java and hence Java users have to cast types manually. We should either create a new method called `javaAssignments` that returns `JavaRDD[(java.lang.Long, java.lang.Int)]` or wrap the result pair in a class. I chose the latter approach in this PR. Now assignments are stored as an `RDD[Assignment]`, where `Assignment` is a class with `id` and `cluster`.

Similarly, in FPGrowth, the frequent itemsets are stored as an `RDD[(Array[Item], Long)]`, which is mapped to `RDD<Tuple2<Object, Object>>`. Though we provide a "Java-friendly" method `javaFreqItemsets` that returns `JavaRDD[(Array[Item], java.lang.Long)]`. It doesn't really work because `Array[Item]` is mapped to `Object` in Java. So in this PR I created a class `FreqItemset` to wrap the results. It has `items` and `freq`, as well as a `javaItems` method that returns `List<Item>` in Java.

I'm not certain that the names I chose are proper: `Assignment`/`id`/`cluster` and `FreqItemset`/`items`/`freq`. Please let me know if there are better suggestions.

CC: jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #4695 from mengxr/SPARK-5900 and squashes the following commits:

865b5ca [Xiangrui Meng] make Assignment serializable
cffa96e [Xiangrui Meng] fix test
9c0e590 [Xiangrui Meng] remove unused Tuple2
1b9db3d [Xiangrui Meng] make PIC and FPGrowth Java-friendly

The API is still not very Java-friendly because `Array[Item]` in `freqItemsets` is recognized as `Object` in Java. We might want to define a case class to wrap the return pair to make it Java friendly.

Author: Xiangrui Meng <meng@databricks.com>

Closes #4661 from mengxr/SPARK-5519 and squashes the following commits:

58ccc25 [Xiangrui Meng] add user guide with example code for fp-growth