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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> The agent that did that analysis tried tagging all the AMDGPU gfx* docs with `:orphan:`, which removes them from the TOC, and that helps a lot.
> 
> I... kind of think the move here is just not to _parse_ the navbar HTML, I don't know why furo does that. If it could just take the HTML as is, or process it with a template engine to customize it instead of reparsing it, that seems like it would go a long way.

FWIW... I posted [llvm/docs/AMDGPU/ contains numerous small, auto-generated files](https://discourse.llvm.org/t/llvm-docs-amdgpu-contains-numerous-small-auto-generated-files/88477) in Oct 2025, but nobody has looked at the problem.

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