Make it easier for developers to find Ballista documentation

[andygrove]

Which issue does this PR close?

Closes #329.

Rationale for this change

See issue.

What changes are included in this PR?

• Added link from top-level README to Ballista README
• Updated links to user guide to point to user guide source for now, until we have a published version
• Removed documentation that is no longer relevant (release process)

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #330 (ba3daba) into master (3f269b9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #330   +/-   ##
=======================================
  Coverage   76.19%   76.19%           
=======================================
  Files         141      141           
  Lines       23786    23786           
=======================================
  Hits        18123    18123           
  Misses       5663     5663           

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[alamb]

Looks good to me -- I clicked around in the Readme's in https://github.com/andygrove/arrow-datafusion/tree/ballista-getting-started and it all looked good to me