Add missing index.md to Pages docs

[ci skip]

doc/user/project/pages/index.md

@@ -444,6 +444,6 @@ For a list of known issues, visit GitLab's [public issue tracker].
 [public issue tracker]: https://gitlab.com/gitlab-org/gitlab-ee/issues?label_name=Pages
 [ce-14605]: https://gitlab.com/gitlab-org/gitlab-ce/issues/14605
 [quick start guide]: ../../../ci/quick_start/README.md
-[pages-index-guide]: ../../../pages/
+[pages-index-guide]: ../../../pages/index.md
 [pages-quick]: ../../../pages/getting_started_part_one.md
 [video-pages-fork]: https://youtu.be/TWqh9MtT4Bg