Update docs to mkdocs

This MR updates the Main branch to use mkdocs to create the documentation rather than sphinx. It does the following:

• changes the contents of .gitlab-cl.yml to point to mkdocs instead of sphinx
• adds mkdocs.yml to configure mkdocs
• changes the contents of the docs folder to reflect the use of mkdocs.

Note that documentation for the code it auto-generated.

The pages will be at the same URL. A preview of the forked pages are here https://ags-python-library-docs-didymograptus-657412b5d7fbb9662ce84a8f8.gitlab.io