[libc][docs] reorder docs to be more beginner friendly (#122376)

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.

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