Add GitHub Actions workflow to extract and update English menu strings in en.yml

[Andygol]

Closes: #66

This PR introduces a new workflow for updating the docs/en/en.yml file with the changes that have occurred in mkdocs.yml, which is then used in Transifex to localise menu items.

Also, along with this workflow, comments were added to mkdocs.yml to explain which menu item is associated with the corresponding line in the file.

The docs/*/*.yml files themselves have also been updated with the latest changes from mkdocs.yml.