Add missing webhook reference anchors in the docs needed by RBTools.
Review Request #12199 — Created March 22, 2022 and submitted
This adds some reference anchors to the Beanstalk, GitHub, and GitHub
Enterprise documentation for linking to the webhook setup documentation.
RBTools tries linking to these references in the Git workflow guide, and
currently this fails. Apparently these never got added to any docs but
Built the docs, and then the RBTools docs. Verified that the links worked.