fix: scope gitignore reference/ rule to repo root only

The bare `reference/` pattern was matching `docs-site/src/reference/`,
preventing documentation files from being tracked without force-add.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako

.gitignore

@@ -33,7 +33,7 @@ reference/mxbuild/
 reference/mendixmodellib/
 reference/mdl-grammar/
 reference/mxbuild-11.6.0/
-reference/
+/reference/
 mx-test-projects/*/
 tmp/