[Docs] Migrate LLVM docs to furo (#184440)
Implements the proposal at
https://discourse.llvm.org/t/rfc-update-llvm-docs-to-furo-theme/90053
Furo output at https://nigham.github.io/llvm_docs_furo/LangRef.html
- Switched theme to Furo
- Removed `:content` directives from RSTs, since Furo always has a TOC
on the side.
Credits for various pieces of feedback and improvements: @Pierre-vh
@aengelke @slinder1 @jhuber6 @fmayer @llvm/infrastructure-area-team
@petrhosek @rnk @boomanaiden154 (see discussions in the RFC)
