FFmpeg install docs: Buanzo.FFmpegforAudacity WinGet package no longer exists #105

In installing-ffmpeg.md:

The referenced package (ID Buanzo.FFmpegforAudacity) was removed in microsoft/winget-pkgs#329673 as the installer (previously rehosted by a third party) wasn't visible to WinGet's validation pipeline.

Another one of the FFmpeg build projects (BtbN/FFmpeg-Builds) publishes to WinGet with IDs BtbN.FFmpeg.*, but the packages:

• Use different naming conventions (different L/GPL packages, packages for different FFmpeg versions, etc.)
• Are laid out differently within the release ZIP (and on-disk)
• Are installed to a different directory (depends on WinGet config)
• Aren't currently autodetected by Audacity

These aren't unsolvable problems, but as it stands there isn't a drop-in replacement within WinGet. Directing folks towards one of the other packages (BtbN.FFmpeg.GPL.Shared.7.1, probably?) seems most reasonable, but the instructions will have to be amended to include locating the dlls.