[SPARK-11399] Add label support in include_example

[yinxusen]

Add label support in include_example.

[SparkQA]

Test build #44899 has finished for PR 9430 at commit ced8b13.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * case class UnresolvedStar(target: Option[Seq[String]]) extends Star with Unevaluable\n

[yinxusen]

@mengxr Until now, all docs of ML and MLlib packages are substituted with include_example except for the DataTypes, mllib-linear-methods and BasicStatistics pages. As we talked before, these three files are dependent on this PR.

Shall we merge this PR so that we can modify all docs before v1.6? Or we defer it to v1.7?

[SparkQA]

Test build #47327 has finished for PR 9430 at commit 3c6ddd8.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[yinxusen]

@mengxr Is the way to add label to include_example good to support the dialog-style docs?

[yinxusen]

I'll close this one according to #11128 for now