Always re-execute example notebooks / use myst-nb

[dweindl]

• Current example notebooks are terribly outdated. Run them on every pull request, to stay in sync with the library.
• Use myst-nb instead of nbsphinx (more actively developed, fewer dependency problems)
• Exclude outdated examples for now
• Use m2r2 instead of unmaintained m2r for markdown parsing

[codecov-commenter]

Codecov Report

Merging #144 (5f5ffdc) into develop (0648b0c) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #144   +/-   ##
========================================
  Coverage    77.73%   77.73%           
========================================
  Files           27       27           
  Lines         2906     2906           
  Branches       718      718           
========================================
  Hits          2259     2259           
  Misses         468      468           
  Partials       179      179           

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