fixed sphinx warning for deprecated source_parsers config field

[telamonian]

References

Code changes

Minor fix for a Sphinx warning that comes up during the docs CI test.

While trying to figure why a docs CI test failed (it was github flood protection during the link check), I noticed this warning:

/opt/hostedtoolcache/Python/3.6.8/x64/lib/python3.6/site-packages/sphinx/util/compat.py:33: RemovedInSphinx30Warning: The config variable "source_parsers" is deprecated. Please update your extension for the parser and remove the setting.
  RemovedInSphinx30Warning)
/opt/hostedtoolcache/Python/3.6.8/x64/lib/python3.6/site-packages/sphinx/util/compat.py:37: RemovedInSphinx30Warning: app.add_source_parser() does not support suffix argument. Use app.add_source_suffix() instead.
  app.add_source_parser(suffix, parser)

I fixed the issue following the markdown setup instructions at https://www.sphinx-doc.org/en/master/usage/markdown.html

User-facing changes

None

Backwards-incompatible changes

None

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[ian-r-rose]

Looks good to me, thanks @telamonian