[clang] [clang-format][CMake] Generate formatting options docs during build (PR #113739)

[Owen Pan via cfe-commits]

owenca wrote:

> As a reviewer that means we have to download the review, rebuild the rst then check it. I don't quite understand the problem we are trying to solve here? Given that the python script unpacks the Format.h and makes the rst I would prefer to see the finished rst as part of the review.

Whenever I review a pull request that has edited Format.h, I always fetch the PR and run the python script to ensure that the generated rst file matches the one in git. I don't think there's a way around that. So I'm ok with reducing the current rst file to a template and removing thousands of lines from the repo.

The alternative would be #111513, which leaves the rst file in tact.

https://github.com/llvm/llvm-project/pull/113739