I’ve been hoping for something like this for soooooo long! 
While I had been toying with the idea of proposing something for almost as long, it was clear that this was a larger project than I could ever hope to do in my freetime.
Now that this is happening, allow me to share some things that I had in mind:
- a version-picker that allows switching between the state of the documentation for various releases (e.g. main, 18.x, 17.x, etc.), with corresponding url scheme – this is standard in many other ecosystems, compare e.g. NumPy (see upper right-hand corner).
- the content behind those should be automatically built from the release branches (resp. tags), not manually by the release managers. The latter means it often doesn’t happen, especially for RCs or later patch releases
- all subprojects should have the same look and feel (if stakeholders can agree…), and ideally, the top-bar should have a ribbon with links to the other subproject pages (and possible highlight for the current one – compare how “API Reference” is highlighted here).
- the content of llvm.org should have the same style, and ideally, be generated from the same repo
- there should be one collected “conformance” page (obviously the content still needs to be sharded). Currently it’s quite a hassle to find the various pages for standards regarding C / C++ / Fortran / OpenMP / etc.
- various nice-to-haves like support for dark-theme, side-TOCs and search functionality, etc.
Even if those features doesn’t end up being part of the GSoC project, I hope we can eventually get to something like a state-of-the-art website, and the consolidation proposed here is a big first step. 