SPARK-3579 Jekyll doc generation is different across environments.

[pwendell]

This patch makes some small changes to fix this problem:

1. We document specific versions of Jekyll/Kramdown to use that match
   those used when building the upstream docs.
2. We add a configuration for a property that for some reason varies across
   packages of Jekyll/Kramdown even with the same version.

[pwendell]

@srowen may want to review this since he recently modified this documentation.

[SparkQA]

QA tests have started for PR 2443 at commit 54ee2ab.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2443 at commit 54ee2ab.

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

[pwendell]

This is a very straightforward change, so I'm going to merge this. However, @srowen feel free to propose tweaks to this in a follow on PR if you think there are any issues.