Updates the shared integrations documentation on generating a URL for
an incoming webhook for the new modal in the web-app, with links to the
help center. Also, links to a new section in the incoming webhooks
overview in the API documentation about the URL specification.
Also, adds a link to the help center in the shared webhooks integration
instructions for creating a stream for an incoming webhook. And updates
the example webhook URL to use a stream ID instead of a stream name.
- 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.