Publish crate docs to GitHub Pages

[bavshin-f5]

docs.rs builds are currently broken (#85), and getting these fixed seems a bit complicated. The next best thing is to host the crate documentation ourselves on GitHub Pages.

Preview: https://bavshin-f5.github.io/ngx-rust/ngx/index.html Only nightly docs for now, will figure out how to handle the tagged releases later.

Required repository configuration:

• Enable pages for the repo (do we need to ask org admin?)
• Settings -> Pages -> Source = Github Actions
• Settings -> Environments -> github-pages -> Deployment branches and tags = master (should be created and configured automatically on the first run).