[clang] [clang][ssaf][docs] Document the Summary Extraction pipeline (PR #172876)

[Balázs Benics via cfe-commits]

steakhal wrote:

> This looks great! The only change I want is to tell the reader that this is rapidly evolving work in progress and that at least initially the documentation will not always be in sync with the code as (just like with this PR) we will use the documentation to supply context to the reviewers of the follow-up PRs with the implementation. If you could add some kind of highlighted message about this to the top of the page that would be great. LGTM otherwise!

<img width="1103" height="259" alt="image" src="https://github.com/user-attachments/assets/a31c1332-43a3-4035-93d2-7558fb763cbc" />
Added a warning message in e4d8c6bfc6e8d95bbd20ba2d47075a095c4b43ca

Also changed from hand rolling out custom static registries and just use `llvm::Registry<>` as the backing implementation.

https://github.com/llvm/llvm-project/pull/172876