And convert its type from `Callable` to `str`.
It's type was previously set to a string in the frontend, and validator
functions in the backend, which were never called.
In preparation for migration to the reference doc.
"Register" and "Registry" are the terms used in the code comments,
but the walkthrough has not used them so far. So, this commit also
introduces these terms in the first paragraph of the section.
Revises the existing API documentation pages for incoming webhooks,
and adds it to the developer documentation at RTD.
Preparation for removing this content from the API documentation,
and redirecting those pages to the new RTD pages.
Co-authored-by: Alya Abbott <alya@zulip.com>
Co-authored-by: Niloth P <20315308+Niloth-p@users.noreply.github.com>
