Fix broken relative links in cookbook docs (Fix #2490)

Cookbook body links referenced sibling pages with a bare slug
(e.g. `c14-keep-alive`). Under the pretty-URL layout each page lives
in its own directory, so these resolve against the page's own
directory and 404. Prefix them with `../` to match the convention
already used in the tour and llm-app sections.

Verified clean with `docs-gen check`.

docs-src/pages/en/cookbook/s13-error-handler.md

@@ -48,4 +48,4 @@ svr.set_error_handler([](const httplib::Request &req, httplib::Response &res) {
 
 Now every error comes back in a consistent JSON shape.
 
-> **Note:** `set_error_handler()` also fires for 500 responses caused by exceptions thrown from a route handler. To get at the exception itself, combine it with `set_exception_handler()`. See [S14. Catch exceptions](s14-exception-handler).
+> **Note:** `set_error_handler()` also fires for 500 responses caused by exceptions thrown from a route handler. To get at the exception itself, combine it with `set_exception_handler()`. See [S14. Catch exceptions](../s14-exception-handler).