[clang-tools-extra] [clang-doc] add async loading (PR #93276)

[Paul Kirth via cfe-commits]

https://github.com/ilovepi approved this pull request.

LGTM modulo some minor nits, but please revise the commit message. The end is not clear, and looks like it may be a copy paste error. If you know how much this improves the performance/responsiveness of the generated documentation, that would also be good to include.

https://github.com/llvm/llvm-project/pull/93276