[157][166] Updated docs for 0.4.0 doc preparation

[jkbradley]

• Removed "SNAPSHOT" label in config.yml for docs
• Updated ruby script for building Python docs to fail more loudly if SPARK_HOME is not present (which I tested manually) and to clean the build (which prevents build failures when built docs already exist)
• Added small clarification to user guide

[jkbradley]

This PR is paired with https://github.com/graphframes/graphframes.github.io/pull/7 which updates the actual github.io documentation.

[felixcheung]

left a note on #179

[codecov-io]

Codecov Report

Merging #180 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #180   +/-   ##
=======================================
  Coverage   88.14%   88.14%           
=======================================
  Files          17       17           
  Lines         717      717           
  Branches       56       56           
=======================================
  Hits          632      632           
  Misses         85       85

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[jkbradley]

I'll merge this now since it seems to have worked. Thanks @felixcheung !