fix more relative links

docs/gitlab/1-2-tools-markdown.md

docs/gitlab/2-3-starting-local-repo.md

docs/gitlab/2-4-starting-git-basics.md

docs/gitlab/3-2-collaborating-fork-their-repo.md

docs/gitlab/3-3-collaborating-play-with-this-page.md

docs/gitlab/repo-doi.md