Document stubs mechanism that is used for FFmpeg

On Android and Linux, FFmpeg has dynamic dependencies, specifically openssl and vaapi (linux only) that we don't deliver with QtMM and which may not be installed on the user PC.
To make the FFmpeg plugin work, we ship stubs of these libs. As a result, if the library is not installed, the FFmpeg only misses the related functionality (unable to play secure streams and missing vaapi hw acceleration).

To make it transparent for users, we should document:

• dependencies. See https://intranet.qt.io/display/QTRD/Shipping+of+shared+FFmpeg+with+dependencies, the image https://intranet.qt.io/download/attachments/218867207/image-2024-4-1_11-46-10.png?version=1&modificationDate=1711964770794&api=v2 describes it pretty well.
• what stubs we ship
• how FFmpeg is patched for working with the stubs (patchelf)
• how to remove the stubs and use direct dependencies (use patchelf again)