docs: Clean redundant relative links.

We previously had a convention of redundantly including the directory
in relative links to reduce mistakes when moving content from one file
to another.  However, these days we have a broken link checker in
test-documentation, and after #21237, MyST-Parser will check relative
links (including fragments) when you run build-docs.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

docs/documentation/openapi.md

@@ -10,7 +10,7 @@ for that file to fully describe every endpoint in the Zulip API, and
 for the Zulip test suite to fail should the API every change without a
 corresponding adjustment to the documentation. In particular,
 essentially all content in Zulip's [REST API
-documentation](../documentation/api.md) is generated from our OpenAPI
+documentation](api.md) is generated from our OpenAPI
 file.
 
 In an OpenAPI Swagger file, every configuration section is an object.