Add local build for docs to contributing

[rogebrd]

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• SDK Code Change
• Example/Test Code Change

Validation

• Does tox pass?
• Do the tests pass?

[codecov]

Codecov Report

Merging #249 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #249   +/-   ##
=======================================
  Coverage   46.98%   46.98%           
=======================================
  Files          32       32           
  Lines       77334    77334           
  Branches     6370     6370           
=======================================
  Hits        36332    36332           
  Misses      40951    40951           
  Partials       51       51           

Flag	Coverage Δ
#unit	46.98% <ø> (ø)

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