Add markdown renderer

[erri120]

Fixes #933, part of #327. This isn't styled, that's part of #943.

out.mp4

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 54.00%. Comparing base (5220371) to head (079c772).
Report is 5 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1112      +/-   ##
==========================================
- Coverage   54.14%   54.00%   -0.14%     
==========================================
  Files         672      673       +1     
  Lines       22270    22311      +41     
  Branches     1709     1713       +4     
==========================================
- Hits        12057    12050       -7     
- Misses       9819     9867      +48     
  Partials      394      394              

Flag	Coverage Δ
Linux	53.40% <0.00%> (-0.09%)	⬇️
Windows	53.31% <0.00%> (-0.09%)	⬇️
clean_environment_tests	54.00% <0.00%> (-0.13%)	⬇️
macOS	52.83% <0.00%> (-0.07%)	⬇️

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Files	Coverage Δ
...ostics/Details/DiagnosticDetailsDesignViewModel.cs	0.00% <0.00%> (ø)
...Pages/Diagnostics/Details/DiagnosticDetailsPage.cs	25.00% <0.00%> (-2.28%)	⬇️
.../Diagnostics/Details/DiagnosticDetailsViewModel.cs	0.00% <0.00%> (ø)

... and 20 files with indirect coverage changes