FFmpeg Download link from Libraries prefs should link to the GitBook page

### Your idea

The FFmpeg **Download** link from Libraries prefs should link to the GitBook page:
https://support.audacityteam.org/basics/downloading-and-installing-audacity/installing-ffmpeg


The user should not be sent to the Manual from this link, nor be sent to this link via the Manual.

### Problem to be solved

Currently the Manual has pages with information on FFmpeg download, it is suspected (from a Forum post) that this not kept up to date.

Since the Manual is planned to be deprecated and superseded by the GitBook support site - and given that the GitBook site already has FFmpeg download information for all three platforms - it makes no sense to maintain the information in both places.

Therefore in the short term I plan to pro tem remove the information in the Manual's pages and replace those with a link to the GitBook page.

This enhancement request is that it makes much more sense for the app to be chanhed so the the FFmmpeg Download button link goes direct to the GitBook page:
https://support.audacityteam.org/basics/downloading-and-installing-audacity/installing-ffmpeg

### Prior art

Not applicable.

### Additional context

Flagging @LWinterberg as the Muse person responsible for documentatioin