[SPARK-21485][FOLLOWUP][SQL][DOCS] Describes examples and arguments separately, and note/since in SQL built-in function documentation

## What changes were proposed in this pull request?

This PR proposes to separate `extended` into `examples` and `arguments` internally so that both can be separately documented and add `since` and `note` for additional information.

For `since`, it looks users sometimes get confused by, up to my knowledge, missing version information. For example, see https://www.mail-archive.com/userspark.apache.org/msg64798.html

For few good examples to check the built documentation, please see both:
`from_json` - https://spark-test.github.io/sparksqldoc/#from_json
`like` - https://spark-test.github.io/sparksqldoc/#like

For `DESCRIBE FUNCTION`, `note` and `since` are added as below:

```
> DESCRIBE FUNCTION EXTENDED rlike;
...
Extended Usage:
    Arguments:
      ...

    Examples:
      ...

    Note:
      Use LIKE to match with simple string pattern
```

```
> DESCRIBE FUNCTION EXTENDED to_json;
...
    Examples:
      ...

    Since: 2.2.0
```

For the complete documentation, see https://spark-test.github.io/sparksqldoc/

## How was this patch tested?

Manual tests and existing tests. Please see https://spark-test.github.io/sparksqldoc

Jenkins tests are needed to double check

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18749 from HyukjinKwon/followup-sql-doc-gen.

## What changes were proposed in this pull request?

This generates a documentation for Spark SQL built-in functions.

One drawback is, this requires a proper build to generate built-in function list.
Once it is built, it only takes few seconds by `sql/create-docs.sh`.

Please see https://spark-test.github.io/sparksqldoc/ that I hosted to show the output documentation.

There are few more works to be done in order to make the documentation pretty, for example, separating `Arguments:` and `Examples:` but I guess this should be done within `ExpressionDescription` and `ExpressionInfo` rather than manually parsing it. I will fix these in a follow up.

This requires `pip install mkdocs` to generate HTMLs from markdown files.

## How was this patch tested?

Manually tested:

```
cd docs
jekyll build
```
,

```
cd docs
jekyll serve
```

and

```
cd sql
create-docs.sh
```

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18702 from HyukjinKwon/SPARK-21485.