Migrate docs to GitHub pages

[chester-leung]

• Changes documentation theme to furo to match Opaque SQL and MC² Client documentation
• Moves documentation to https://mc2-project.github.io/secure-xgboost/
• Adds one GitHub action to test documentation build on every PR, and a second GitHub action to automatically publish documentation updates on every push to master

[chester-leung]

Closing this and reopening due to a bug with GitHub actions' inability to detect new workflows