[llvm] [Docs] Migrate LLVM docs to furo (PR #184440)

[Alexis Engelke via llvm-commits]

aengelke wrote:

Seconding @boomanaiden154 -- not just for instructions, but also for intrinsics.

InstCombineContributorGuide TOC is read.

Also, probably as a separate patch (preferably before this PR), could we restructure our document hierarchy for the left side bar? Having LangRef buried rather deeply under arguably-less-important docs like CallGraphSection isn't ideal. The structure generally doesn't seem to make much sense (e.g. HowToSetUpLLVMStyleRTTI is under reference not user guides, DiscourseMigrationGuide is top-level) and should be cleaned up as it is now a lot more visible than before.

Also in the left sidebar, the four items below SandboxIR link to the same page and should be indented. (Something similar can be observed for other pages, too.) Is it possible to fix all these cases?
<img width="226" height="346" alt="image" src="https://github.com/user-attachments/assets/0fe56827-47e5-48d3-9dc2-d715c8c62e94" />
Also weird that "Great, what do I need to do!" is such a prominent entry... :joy: 
<img width="257" height="411" alt="image" src="https://github.com/user-attachments/assets/e9f3f916-8c92-4bad-915c-e2a92831d20c" />



https://github.com/llvm/llvm-project/pull/184440