docs: replace recommonmark with myst-parser in docs configuration

[MWill727]

Fixes

Replacing recommonmark with myst_parser in the Sphinx docs configuration and updating the related test dependency to myst-parser, since recommonmark is legacy and myst_parser is the actively maintained Markdown parser recommended for Sphinx.

Verified in a virtual environment that myst_parser installs and imports successfully.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

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