Move documenation to MyST.

[tobiasraabe]

Changes

This PR moves the documentation to MyST because

• for most people it is easier to write markdown.
• https://mdformat.readthedocs.io/ allows to format the documentation although we cannot use it right now since it cannot handle admonitions. we keep an eye on it.

[codecov]

Codecov Report

Merging #232 (529a3a8) into main (3e41ebd) will decrease coverage by 0.02%.
The diff coverage is 60.00%.

❗ Current head 529a3a8 differs from pull request most recent head dcd7e1d. Consider uploading reports for the commit dcd7e1d to get more accurate results

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
- Coverage   94.37%   94.35%   -0.03%     
==========================================
  Files          79       79              
  Lines        6956     6961       +5     
==========================================
+ Hits         6565     6568       +3     
- Misses        391      393       +2     

Flag	Coverage Δ
end_to_end	80.17% <60.00%> (-0.02%)	⬇️
integration	41.74% <20.00%> (-0.02%)	⬇️
unit	69.38% <20.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/_pytask/capture.py	90.33% <ø> (ø)
src/_pytask/mark/expression.py	98.36% <ø> (ø)
tests/test_graph.py	95.60% <60.00%> (-2.08%)	⬇️

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