Build site using GitHub Actions

[mborgerson]

This only builds; does not deploy.

• angr.io: is served by github pages from the angr.github.io repo root directory
• api.angr.io: is a redirect to angr.io/api-docs, served from github pages
• docs.angr.io: is served by hosting.gitbook.io, apparently just sourced from https://github.com/angr/angr-doc

I think it would be simpler to have the generated API docs served separately from the generated main site content (e.g. site content being on gh-pages branch); which would avoid need for repository synchronization and needing a token to push to a remote repository. -- I've deferred deployment step for that conversation.

Partial solution to #21

[rhelmot]

I already consider api.angr.io to be the canonical api-doc url, so I think it's fine if we switch to that