Move to GH Actions for build and deploy

[alexeyzimarev]

The website is currently being built and deployed by Cloudflare integration, which makes build logs invisible. It's hard to debug failing builds, users are required to have access to Cloudflare, so external contributions are troublesome.

We should move to GitHub Actions for Cloudflare Pages so the build runs locally.

Another things to remember is to delete deployments for deleted branches and closed PRs. Cloudflare doesn't do it.