Add document for universal build environments and installation methods

[Atry]

These documents are basically copied from https://hhvm.com/blog/2022/07/12/experimenting-with-nix-github-actions-and-visual-studio-code.html and https://hhvm.com/blog/2022/08/30/experimenting-with-universal-deb-packages.html, with some modification about clang packages.

Test Plan:

Visit http://docs-prod-nix-lb-146144460.us-west-2.elb.amazonaws.com/hhvm/experimental-builds/introduction for preview