Automate a spell checker for the documentation

[tkknight]

Use https://sphinxcontrib-spelling.readthedocs.io/en/latest/ in the conf.py for sphinx.

• Spell checker for the sphinx documentation (not enabled for now) #3779 to correct spelling and put in place the spelling framework (but not enable atm)
• Docs: Enable spell check #3882 to enable the automated spell check once the dependencies are resolved. Need to:
  • uncomment conf.py extension reference
  • uncomment Travis sub job
  • Test Travis sub job (check a spelling warning will cause the job to fail)
  • uncomment text in contributing_documentation.rst

[tkknight]

Closing this issue, #3882 will remain.