Migrating docs to GitHub Actions to fix broken link

[komailo]

• Migrating docs build and publish to GitHub Actions
• Base URL in docs/_config.yml had the full link, dropped the protocol and hostname to allow for forks to build and publish the docs.
• Leaving the GitLab CI file in docs as is, in case its used internally

Tested in my fork: https://komailo.github.io/shadowenv/

Fixes: #113

This is my first PR in the repo, if the docs are published by another CI, happy to investigate it out. Let me know.

[komailo]

I have signed the CLA!