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/subsystems/html-css.md

@@ -145,7 +145,7 @@ developing new features for Zulip that require front-end changes,
 especially those that involve adding new files. For a more general
 overview, see the [new feature tutorial](../tutorials/new-feature-tutorial.md).
 
-Our [dependencies documentation](../subsystems/dependencies.md) has useful
+Our [dependencies documentation](dependencies.md) has useful
 relevant background as well.
 
 ### Primary build process