Update the legacy starter pack for sphinx-docs

Currently COU docs are failing in the CI and one of the links `https://library.canonical.com/documentation/read-the-docs` are not public anymore and need SSO to access it.

There are documentation on how to update the docs here and we need to update to not be outdated and with the CI broken. Instructions can be found [here](https://github.com/canonical/sphinx-docs-starter-pack/blob/3d44be8dfc7b8d8f218c59d7b91f3cbe991736cc/docs/how-to/update-starter-packs/legacy-starter-pack.rst#id1)

Start with the legacy starter pack instructions linked in the issue, then inspect the COU documentation CI failure and the inaccessible library.canonical.com link. Done means the starter pack guidance is current, the non-public link is addressed, and the documentation CI passes.

Written by the indexing model from the issue text.