Update mkdocs.yml 'edit_uri' for easy web editing

The existing edit_uri value in mkdocs.yml disables easy editing of pages.

To facilitate Guidebook contributions, I'd like to enable easy access of the Gitlab web editor, so specific pages can be more conveniently updated or contributed to, using the Gitlab web editor.

This means anyone logged into Gitlab already who has sufficient access to create MRs can offer edits via the web editor, whereas unauthorised users gain no additional access.

Configuration details can be found in the official MkDocs documentation here, under edit_uri section linked, and the subsequent edit_uri_template section:

https://www.mkdocs.org/user-guide/configuration/#edit_uri

Additional Notes

• Whichever user is web editing must know not to commit directly to master branch, and instead to create a branch in the web editor then a merge request from there (which the web editor UI facilitates).
• Existing branch protection rules on this project should make direct commit to master impossible, including by the web editor, but the button for a commit to master is still visible.