docs: ReplayGain Plugin: Add details how to use GStreamer on Ubuntu

[milnak]

I've found that ReplayGain plugin has issues in default config (see #5449 ) so I decided to use GStreamer which seems to work, however, there's no documentation on how to set up and use GStreamer on Ubuntu.

Here's how I got it to work:

# Install gstreamer and PyGObject (python-gi)
sudo apt-get install libcairo2-dev libgirepository-2.0-dev gstreamer-1.0 gstreamer1.0-tools gstreamer1.0-plugins-* python3-gi --yes
pip install PyGObject

Then modify the beets/config.yaml to have (for example):

plugins: replaygain
replaygain:
    backend: gstreamer

and then run

beet --verbose  replaygain --write