[techdocs] Use `gh-admonitions` plugin everywhere

The `gh-admonitions` plugin was added to the shared workflow in https://github.com/grafana/shared-workflows/commit/7973f46778ca0748974cb2c38670300b6efc57d0

It's not ideal that individual repositories have to add this to their config. This ought to be something that works out ouf the box.

The `@techdocs/cli` script has a flag:

```
Usage: techdocs-cli generate|build [options]

Generate TechDocs documentation site using MkDocs.

Options:
...
--defaultPlugin [defaultPlugins...] Plugins which should be added automatically to the mkdocs.yaml file (default: [])
...
```

which might be able to be used for this purpose, if it works like it sounds.