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

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:

```shell
# 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):

```yaml
plugins: replaygain
replaygain:
backend: gstreamer
```

and then run

```shell
beet --verbose replaygain --write
```

The issue names no documentation file; start by locating the ReplayGain plugin documentation and any existing backend setup guidance. Add the Ubuntu GStreamer installation commands, beets/config.yaml example, and replaygain command from the report, then verify that the instructions are complete and readable.

Written by the indexing model from the issue text.