[SPARK-10689] [ML] [Doc] User guide and example code for AFTSurvivalRegression

[yanboliang]

Add user guide and example code for AFTSurvivalRegression.

[SparkQA]

Test build #45112 has finished for PR 9491 at commit 5f4f6d8.

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

[yanboliang]

I think the best way is to load some sample data into DataFrame like LiR and LoR, but I found it not very appropriate because:
If the data stored as text file, we need to define one case class like LabeledPoint which may confuse users;
If the data stored as Parquet/ORC file, users can not explore the data intuitively;
If the data stored as JSON, I think it's the best suitable way, but DataFrame can not load data with Vector type correctly.

[mengxr]

@yanboliang We implemented a new approach for example code in user guide. Could you use include_example instead? This is an example: 820064e.

[SparkQA]

Test build #45211 has finished for PR 9491 at commit ff9ddb9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * public class JavaAFTSurvivalRegressionExample\n

[mengxr]

line too wide. Maybe we didn't turn on style check for code under examples/.

[SparkQA]

Test build #45352 has finished for PR 9491 at commit b36401d.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * public class JavaAFTSurvivalRegressionExample\n

[mengxr]

LGTM. Merged into master and branch-1.6. Thanks!