[SPARK-14817][ML][MLLIB][DOC] Made DataFrame-based API primary in MLlib guide

[jkbradley]

What changes were proposed in this pull request?

Made DataFrame-based API primary

• Spark doc menu bar and other places now link to ml-guide.html, not mllib-guide.html
• mllib-guide.html keeps RDD-specific list of features, with a link at the top redirecting people to ml-guide.html
• ml-guide.html includes a "maintenance mode" announcement about the RDD-based API
  • Reviewers: please check this carefully
• (minor) Titles for DF API no longer include "- spark.ml" suffix. Titles for RDD API have "- RDD-based API" suffix
• Moved migration guide to ml-guide from mllib-guide
  • Also moved past guides from mllib-migration-guides to ml-migration-guides, with a redirect link on mllib-migration-guides
  • Reviewers: I did not change any of the content of the migration guides.

Reorganized DataFrame-based guide:

• ml-guide.html mimics the old mllib-guide.html page in terms of content: overview, migration guide, etc.
• Moved Pipeline description into ml-pipeline.html and moved tuning into ml-tuning.html
  • Reviewers: I did not change the content of these guides, except some intro text.
• Sidebar remains the same, but with pipeline and tuning sections added

Other:

• ml-classification-regression.html: Moved text about linear methods to new section in page

How was this patch tested?

Generated docs locally

[SparkQA]

Test build #62360 has finished for PR 14213 at commit 00c9941.

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

[jkbradley]

CC: @mengxr @dbtsai @MLnick @yanboliang @hhbyyh @thunterdb CCing many since this is a major doc change

[jkbradley]

Since up-to-date docs will help with people testing the RC, and a new RC is being cut soon, I'm going to merge this. However, please review it still, and I can send follow-up patches. Thanks all.

[jkbradley]

I'm going to go ahead and give this a LATM (Looks Amazing To Me) and close the JIRA.