tests: Split `test_doc_endpoints` into more specific endpoint tests.
tests: Split `test_doc_endpoints` into more specific endpoint tests.

In `zerver/tests/test_docs.py`, we split the catch all test for doc
endpoints into more specific test groups: dev environment pages,
error pages, corporate pages. Also, moves the api endpoints being
tested to the specific test for api endpoints.

andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip