[SPARK-25584][ML][DOC] datasource for libsvm user guide

[zhengruifeng]

What changes were proposed in this pull request?

it seems that doc for libsvm datasource is not added in #22675.
This pr is to add it.

How was this patch tested?

doc built locally

[zhengruifeng]

The orginal url: ml-datasource is incorrect that the generated url lose suffix .html
This do not matter in the offical web, since it seems that model web browser will automatically add the suffix.
However, in the locally built docs, we can not open the ml-datasource link file:///Users/xxx/Dev/OpenSource/spark/docs/_site/ml-datasource from page file:///Users/xxx/Dev/OpenSource/spark/docs/_site/ml-guide.html.

[zhengruifeng]

ping @mengxr @WeichenXu123 ? would you mind help reviewing this? thanks in advance!

[SparkQA]

Test build #108321 has finished for PR 25286 at commit 0e23538.

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

[SparkQA]

Test build #108323 has finished for PR 25286 at commit 8cf823a.

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

[HyukjinKwon]

(ignore the logs above. I was testing merge script Python 3 compatibility)

[srowen]

Merged to master