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

In [installing-ffmpeg.md](https://github.com/audacity/audacity-support/blob/0f3f24773e8bfa0a2984070cabf96707bbbd072c/basics/installing-ffmpeg.md?plain=1#L35-L39):
https://github.com/audacity/audacity-support/blob/0f3f24773e8bfa0a2984070cabf96707bbbd072c/basics/installing-ffmpeg.md?plain=1#L35-L39

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`](https://github.com/BtbN/FFmpeg-Builds/releases)) 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.

Start in basics/installing-ffmpeg.md around lines 35-39 and verify the current WinGet package and release layout for the proposed BtbN package. Update the instructions so they no longer reference the removed package, explain how to locate the required DLLs, and confirm that the documented steps reflect Audacity's current detection behavior.

Written by the indexing model from the issue text.