add mvp of spelling sphinx extension

[rwegener2]

What has been built?

This PR adds MVP-level functionality to use the sphinx spelling extension as a spell checker.

How was it done?

• sphinxcontrib.spelling was added as an extension in _conf.yml
• documentation for how to run the spell check were added to the README
• a sheet called spelling_correctwords.txt was added to the root that contains words that should be skipped by spell check

How can it be tested?

Run the instructions below (copied from the README):

• pip install sphinxcontrib-spelling -- this installs the proper module in the environment
• jupyter-book config sphinx generates conf.py -- this creates a file (conf.py) that will allow us to bypass jupyter-book build . and run the sphinx build command directly
• Add the following lines to conf.py: -- these are our custom configuration settings for the module

spelling_ignore_pypi_package_names=True
spelling_word_list_filename = 'spelling_correctwords.txt'

• sphinx-build -b spelling . ./_build from root (this is in place of jupyter-book build . -- This builds the book while checking for spelling. The output of this build includes locations of spelling errors detected in the text