Add ffmpeg reference docs

[IanButterworth]

Built on #433

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.82%. Comparing base (0431628) to head (d2f8e3e).
⚠️ Report is 1 commits behind head on ffmpeg8.

Additional details and impacted files

@@           Coverage Diff            @@
##           ffmpeg8     #435   +/-   ##
========================================
  Coverage    77.82%   77.82%           
========================================
  Files           10       10           
  Lines         1308     1308           
========================================
  Hits          1018     1018           
  Misses         290      290           

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