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

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

It looks like LangRef currently has an additional level in the table of contents:

<img width="205" height="187" alt="Image" src="https://github.com/user-attachments/assets/aff9a69b-2da9-486e-888c-ff1dc7e18fb9" />

<img width="499" height="161" alt="Image" src="https://github.com/user-attachments/assets/98c4564f-40b9-4966-9a9c-3be9e029d26d" />

Is it possible to drop that last level? It makes the ToC quite a bit less useable and that is definitely the ToC I use the most.

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