[clang] [clang][docs] Generate diagnostics reference as Markdown (PR #221361)

[Reid Kleckner via cfe-commits]

rnk wrote:

> Really nice! Very happy to see my rST table generator getting deleted :) I think the markdown output is much less readable, but I don't believe anyone is reading it anyway, so that's not really a loss.

Glad to hear it! Your grid-table C++ emission was compact and elegant in part due to its inefficiency of recursively building up `std::string` table cells. FP style really is the best for comprehension. I feel like I'm replacing that artisanal hack with very bland code.

> I did a spot check and mostly the output seems to be identical to the old output, except in cases like `-Wc++11-narrowing` ([before](https://llvmdocs.staging.reidkleckner.dev/before/clang/docs/DiagnosticsReference.html#wc-11-narrowing), [after](https://llvmdocs.staging.reidkleckner.dev/after/clang/docs/DiagnosticsReference.html#wc-11-narrowing)) where this fixes a bug. (There are a bunch more instances of this, all similar: the rST output was incorrectly centering things that should have been left-aligned when a table cell contains a placeholder.) The rendering of `-Wframe-larger-than`'s description is also improved here.

Yeah, I think this was a result of killing single-celled tables:
<img width="1265" height="845" alt="image" src="https://github.com/user-attachments/assets/d5ed1829-3889-440f-be0d-9282c56de5f0" />


https://github.com/llvm/llvm-project/pull/221361