[clang-tools-extra] [clang-tidy] Generate Markdown documentation for new checks (PR #210574)

[Zeyi Xu via cfe-commits]

zeyi2 wrote:

I'm not sure a dedicated test is necessary for this PR. Previous changes to this script generally haven't added tests, and [update-checks-list.test](https://github.com/llvm/llvm-project/blob/main/clang-tools-extra/test/clang-tidy/infrastructure/update-checks-list.test) should catch regressions in the generated check list.

I also tested this PR in a fresh worktree with #210466 and #210467 applied. `--update-docs` produced no changes, and adding a new check correctly generated its Markdown documentation and updated the check list.

https://github.com/llvm/llvm-project/pull/210574