[SPARK-14514][DOC] Add python example for VectorSlicer

[zhengruifeng]

What changes were proposed in this pull request?

Add the missing python example for VectorSlicer

How was this patch tested?

manual tests

[SparkQA]

Test build #55465 has finished for PR 12282 at commit 1b139c7.

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

[holdenk]

So this one differs slightly from the Scala example, it might make sense to have a follow JIRA for once/if AttributeGroup is exposed in Python to also update this example. What do you think?

[zhengruifeng]

@holdenk I agree with you. Python wrapper for AttributeGroup is missing now.

[holdenk]

Rad, maybe @jkbradley can take a look at this PR?

[jkbradley]

test this please

[jkbradley]

LGTM pending an updated run of the tests

I agree we can update this later after Attributes are exposed in Python. We want to re-evaluate the attribute/metadata API first.

[SparkQA]

Test build #57027 has finished for PR 12282 at commit 1b139c7.

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

[jkbradley]

Merging with master
Thanks @zhengruifeng !