[SPARK-4771][Docs] Document standalone cluster supervise mode

tdas looks like streaming already refers to the supervise mode. The link from there is broken though.

Author: Andrew Or <andrew@databricks.com>

Closes #3627 from andrewor14/document-supervise and squashes the following commits:

9ca0908 [Andrew Or] Wording changes
2b55ed2 [Andrew Or] Document standalone cluster supervise mode
SPARK-3526 Add section about data locality to the tuning guide

cc kayousterhout

I have a few outstanding questions from compiling this documentation:
- What's the difference between NO_PREF and ANY?  I understand the implications of the ordering but don't know what an example of each would be
- Why is NO_PREF ahead of RACK_LOCAL?  I would think it'd be better to schedule rack-local tasks ahead of no preference if you could only do one or the other.  Is the idea to wait longer and hope for the rack-local tasks to turn into node-local or better?
- Will there be a datacenter-local locality level in the future?  Apache Cassandra for example has this level

Author: Andrew Ash <andrew@andrewash.com>

Closes #2519 from ash211/SPARK-3526 and squashes the following commits:

44cff28 [Andrew Ash] Link to spark.locality parameters rather than copying the list
6d5d966 [Andrew Ash] Stay focused on Spark, no astronaut architecture mumbo-jumbo
20e0e31 [Andrew Ash] SPARK-3526 Add section about data locality to the tuning guide

(cherry picked from commit 652b781a9b543cb17d7da91f5c3bebe5a02e0478)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>