Creates a new toctree "Support" under which we have distinct links to arch,
platform, and compiler support.
* Moved "Platform Support" from index landing page to new doc.
* Created explicit "Architecture Support". Requested in https://github.com/llvm/llvm-project/issues/118964#issuecomment-2531503046.
* Moved "Compiler Support" from Status toctree to new Support toctree.
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Co-authored-by: Carlo Cabrera <github@carlo.cab>
