Build docs with myst instead of rst

[jacobtomlinson]

Experiment with using myst instead of rst.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@78e914b). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
sphinx_helm/ext.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #5   +/-   ##
=======================================
  Coverage        ?   73.97%           
=======================================
  Files           ?        6           
  Lines           ?      196           
  Branches        ?        0           
=======================================
  Hits            ?      145           
  Misses          ?       51           
  Partials        ?        0           

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