Updates `.md` files in api_docs/ to use "stream" instead of "channel"
for descriptive text, with the exception of the API changelog file.
Part of stream to channel rename project.
- 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.
Lauryn Menard
David Rosa
Alex Vandiver