This commit does a few things, with the intent to make it more straightforward
to potential future users how to get started with llvm-libc. Answers to "What's
the status and how do I use it?" are front and center, "above the fold."
This commit does a few things:
* reorganize the landing page's toctree: start with implementation status, arch
* support, platform support, and
compiler support.
* Then a (new) simple getting started page using full host builds. Then more
* Advanced topics such as host vs cross builds, overlay mode,
gpu builds and additional configuration options.
* Remove c23 status, the old fullbuild_mode landing page, and
usage_modes landing pages. c23 status isn't as interesting as I originally
thought it might.
We should point people at full host builds to start, then link to more info on
cross compilation, or overlay mode as more advanced topics. I assert most users
starting out won't care about those.
Since new headergen is now the default for building LLVM-libc, the docs
need to be updated to reflect that. While I was editing those docs, I
took a quick pass at updating other out-of-date pages.
