[SPARK-9671] [MLLIB] re-org user guide and add migration guide

This PR updates the MLlib user guide and adds migration guide for 1.4->1.5.

* merge migration guide for `spark.mllib` and `spark.ml` packages
* remove dependency section from `spark.ml` guide
* move the paragraph about `spark.mllib` and `spark.ml` to the top and recommend `spark.ml`
* move Sam's talk to footnote to make the section focus on dependencies

Minor changes to code examples and other wording will be in a separate PR.

jkbradley srowen feynmanliang

Author: Xiangrui Meng <meng@databricks.com>

Closes #8498 from mengxr/SPARK-9671.