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

## 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

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #14213 from jkbradley/ml-guide-2.0.

1 files changed, 2 insertions, 2 deletions

diff --git a/docs/index.md b/docs/index.md
index 7157afc411..0cb8803783 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -8,7 +8,7 @@ description: Apache Spark SPARK_VERSION_SHORT documentation homepage
 Apache Spark is a fast and general-purpose cluster computing system.
 It provides high-level APIs in Java, Scala, Python and R,
 and an optimized engine that supports general execution graphs.
-It also supports a rich set of higher-level tools including [Spark SQL](sql-programming-guide.html) for SQL and structured data processing, [MLlib](mllib-guide.html) for machine learning, [GraphX](graphx-programming-guide.html) for graph processing, and [Spark Streaming](streaming-programming-guide.html).
+It also supports a rich set of higher-level tools including [Spark SQL](sql-programming-guide.html) for SQL and structured data processing, [MLlib](ml-guide.html) for machine learning, [GraphX](graphx-programming-guide.html) for graph processing, and [Spark Streaming](streaming-programming-guide.html).
 
 # Downloading
 
@@ -87,7 +87,7 @@ options for deployment:
 * Modules built on Spark:
   * [Spark Streaming](streaming-programming-guide.html): processing real-time data streams
   * [Spark SQL, Datasets, and DataFrames](sql-programming-guide.html): support for structured data and relational queries
-  * [MLlib](mllib-guide.html): built-in machine learning library
+  * [MLlib](ml-guide.html): built-in machine learning library
   * [GraphX](graphx-programming-guide.html): Spark's new API for graph processing
 
 **API Docs:**