[llvm] [docs] Check for absolute self-documentation links (PR #222504)

[Aiden Grossman via llvm-commits]

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boomanaiden154 wrote:

I didn't realize that this script was a thing. It would be nice if we wired this up into lit somewhere, but I guess that's a separate change and made harder by the fact this is in top-level `utils/`.

https://github.com/llvm/llvm-project/pull/222504