Disabling Sphinx mermaid extension, which breaks with Sphinx 4.0+.

[jeff-cohere]

Our most recent attempt at auto-publishing our documentation site failed with this error:

...
Running Sphinx v4.0.1

Extension error:
Could not import extension sphinxcontrib.mermaid (exception: cannot import name 'ENOENT' from 'sphinx.util.osutil' (/usr/local/lib/python3.8/site-packages/sphinx/util/osutil.py))
make: *** [Makefile:19: html] Error 2
...
[sphinx-action] Starting sphinx-action build.
Running: pip install -r requirements.txt
====================================
Building docs in doc/sphinx
====================================
[sphinx-action] Running: ['make', 'html', '-e']
[sphinx-action] Build failed with 0 warnings
Traceback (most recent call last):
  File "/entrypoint.py", line 22, in <module>
    action.build_all_docs(github_env, [os.environ.get("INPUT_DOCS-FOLDER")])
  File "/sphinx_action/action.py", line 167, in build_all_docs
    raise RuntimeError("Build failed")
RuntimeError: Build failed

It seems like Sphinx 4.0 has removed sphinx.util.osutil.ENOENT, and the Mermaid extension contributors haven't gotten the memo yet. I don't think we're using Mermaid anywhere just yet, so I've removed it from our configuration. We'll see if this fixes the issue.

[codecov-commenter]

Codecov Report

Merging #173 (157f982) into master (9be9df5) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #173   +/-   ##
=======================================
  Coverage   67.73%   67.73%           
=======================================
  Files           7        7           
  Lines        1221     1221           
=======================================
  Hits          827      827           
  Misses        394      394           

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