[clang] [CIR][docs] C++ cleanup and exception handling design for CIR (PR #177625)

[Reid Kleckner via cfe-commits]

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rnk wrote:

You have the option to use markdown instead of restructured text, if you prefer.

I went with markdown when doing the AI policy doc in part because it made it easy to copy-paste into Discourse, which takes markdown, and I think that doc has examples of any and all the Sphinx tags you might care about.

I think I used [Pandoc](https://pandoc.org/) to do the initial conversion, and cleaned it up from there.

We don't have project wide policy on this, but there's been a steady move in the direction of converting rst to markdown, see the [release notes conversion RFC](https://discourse.llvm.org/t/rfc-convert-llvms-release-notes-to-markdown/81307/6).

https://github.com/llvm/llvm-project/pull/177625