[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.
7 files changed
tree: 48e8a9c7aab30f9714d366b07b754b9e2da80780
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. libc/
  11. libclc/
  12. libcxx/
  13. libcxxabi/
  14. libunwind/
  15. lld/
  16. lldb/
  17. llvm/
  18. llvm-libgcc/
  19. mlir/
  20. offload/
  21. openmp/
  22. polly/
  23. pstl/
  24. runtimes/
  25. third-party/
  26. utils/
  27. .clang-format
  28. .clang-tidy
  29. .git-blame-ignore-revs
  30. .gitattributes
  31. .gitignore
  32. .mailmap
  33. CODE_OF_CONDUCT.md
  34. CONTRIBUTING.md
  35. LICENSE.TXT
  36. pyproject.toml
  37. README.md
  38. SECURITY.md
README.md

The LLVM Compiler Infrastructure

OpenSSF Scorecard OpenSSF Best Practices libc++

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