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

[podcastinator]

Thanks!

1. Are you able to change the color scheme for the "Note" boxes? The light blue links are not particularly visible on the orange background.
2. Sub-bullets render weirdly on the following pages: https://mc2-project.github.io/secure-xgboost/tutorials/outsourced.html https://mc2-project.github.io/secure-xgboost/tutorials/distributed.html https://mc2-project.github.io/secure-xgboost/tutorials/multiclient-distributed.html
3. The following pages have errors: https://mc2-project.github.io/secure-xgboost/parameter.html https://mc2-project.github.io/secure-xgboost/python/python_api.html

[chester-leung]

Thanks for catching these. I've changed the orange background to a light gray.

I don't think these most recent changes will be reflected on the mc2-project.github.io site, as changes are only pushed to that once they're merged into master. You may have to build everything locally to view it.

We should probably also set something up that'll allow us to view documentation changes before they're merged. I'll add a note for this.

[podcastinator]

Ok, merging this for now, haven't tested locally.