[techdocs] Use gh-admonitions plugin everywhere

[iainlane]

The gh-admonitions plugin was added to the shared workflow in 7973f46

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.