feat: Enable versioning with MkDocs plugin 'mike'

• Update mkdocs.yml to include 'mkdocs build' parameters & specify theme overrides dir
• Add theme override to display notice when browsing non-latest docs & link to latest
• Update Gitlab CI config to improve neatness, reduce duplication, add 'mike' scripts
• Update README.md to describe 'mike' usage including Gitlab project access token

Separately, these required manual steps have been performed:

• Created project access token & added in CI/CD Variables ( $PROJECT_ACCESS_TOKEN )
• Pushed branch pages which currently matches master

Closes #261