FFmpeg installations are not documented

[jkwmoore]

We have installs on both clusters, but there's no mention in the docs.

Stanage:

module spider ffmpeg

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  FFmpeg:
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    Description:
      A complete, cross-platform solution to record, convert and stream audio and video.

     Versions:
        FFmpeg/4.2.2-GCCcore-9.3.0
        FFmpeg/4.3.1-GCCcore-10.2.0
        FFmpeg/4.3.2-GCCcore-10.3.0
        FFmpeg/4.3.2-GCCcore-11.2.0
        FFmpeg/4.4.2-GCCcore-11.3.0

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  For detailed information about a specific "FFmpeg" package (including how to load the modules) use the module's full name.
  Note that names that have a trailing (E) are extensions provided by other modules.
  For example:

     $ module spider FFmpeg/4.4.2-GCCcore-11.3.0
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Bessemer:

modulefind FFmpeg
FFmpeg/4.2.2-GCCcore-9.3.0