[clang] Move documentation about -verify from a header to public docs (PR #73694)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman After moving the docs, none of the code blocks are rendering: https://clang.llvm.org/docs/InternalsManual.html#verifying-diagnostics

huh? there are zero sphinx diagnostics about the blocks, so that's exciting.

https://github.com/llvm/llvm-project/pull/73694