Adds a page to the general api documentation about HTTP headers,
so that information about the special response headers for rate
limits have a more logical location in the docs and so that other
HTTP header information can be shared, such as `User-Agent`
conventions.
Adjusts some text and linking on the rest-error-handling page and
overview page for the REST API for the addition of the HTTP headers
page.
- Updates `.prettierignore` for the new directory.
- Updates any reference to the API documentation directory for
markdown files to be `api_docs/` instead of `zerver/api/`.
- Removes a reference link from `docs/documentation/api.md` that
hasn't referenced anything in the text since commit 0542c60.
- Update rendering of API documentation for new directory.