Move api docs into GitHub pages

[GuySartorelli]

The API docs are a bunch of static files generated on a cron.

Right now we're hosting them on silverstripe cloud, despite this not being a Silverstripe CMS site. Which is silly.

We should move the API docs into GitHub pages and set them up to rebuild every x amount of time via a GitHub Actions cron.

Acceptance Criteria

• api.silverstripe.org is hosted on GitHub Pages
• The search functionality used by API links in the docs is retained as closely as sensibly possible
• The site can be built manually, is build when the master branch is pushed, and is built on a cron
• After hosting is swapped over, the old stack in Silverstripe Cloud is removed

API doc PR

• ENH Prepare codebase to migrate to GitHub Pages #127
• Revert "Merge pull request #127 from creative-commoners/pulls/master/… #128

After merging, reassign to Guy to set up GitHub Pages here and organise swapping hosting over.

PRs for after hosting has swapped over

This PR MUST NOT be merged until API docs hosting has swapped over.

• FIX Update API links URL for new hosting doc.silverstripe.org#317

[GuySartorelli]

Not doing this anymore. New card will be created with the preferred alternative.