Change link to relative, compile-checked link.

Remove absolute link (which replaced a Jekyll link tag with a syntax error).
Links which only link to other sections of the community pages should use this
syntax, so they are checked for validity at build time.

docs/CONTRIBUTING.md

@@ -37,4 +37,4 @@ We invite you to review our [roadmap]({% link docs/community/roadmap.md %}) and
 If you are confident using git, markdown, and GitLab pages, you are very welcome to [submit an issue to our GitLab repository](https://git.fmrib.ox.ac.uk/open-science/community/Open-WIN-Community/-/issues), and submit a merge request.
 
 ### New to GitLab?
-If you'd like to try out GitLab at your own pace, take a look at the [training materials available here](https://open.win.ox.ac.uk/pages/open-science/community/Open-WIN-Community/docs/gitlab/).
+If you'd like to try out GitLab at your own pace, take a look at the [training materials available here]({% link docs/gitlab.md %}).