Include magics README in docs

[blink1073]

Adds a Magics Reference page to the MkDocs site by including metakernel/magics/README.md via the snippets plugin. Also excludes docs/magics.md from mdformat to prevent it from escaping the snippet syntax.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.97%. Comparing base (1988e3c) to head (fc32269).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #367   +/-   ##
=======================================
  Coverage   90.97%   90.97%           
=======================================
  Files          51       51           
  Lines        2903     2903           
  Branches      408      408           
=======================================
  Hits         2641     2641           
  Misses        183      183           
  Partials       79       79           

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