Add FAQ page to docs

[djhoese]

Adds an initial FAQ and performance tips page to the sphinx docs.

[coveralls]

Coverage remained the same at 77.452% when pulling b4cb681 on djhoese:doc-faq into 73c1600 on pytroll:master.

[codecov]

Codecov Report

Merging #590 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #590   +/-   ##
=======================================
  Coverage   77.45%   77.45%           
=======================================
  Files         136      136           
  Lines       19168    19168           
=======================================
  Hits        14846    14846           
  Misses       4322     4322

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