As part of https://gitlab.com/gitlab-org/gitaly/-/issues/6388, we've
identified several places where Gitaly's architecture is documented.

Customers and customer support are more likely to read about Gitaly
through the official documentation rather than this README. There's no
point in maintaining the same information in both places.

Remove the architecture diagrams and list of Gitaly clients from the
README, and add a link to the documentation instead.