Merge branch 'docs/mermaid-gl-version' into 'master'

Add version that the mermaid support was introduced

See merge request gitlab-org/gitlab-ce!15688

doc/user/markdown.md

@@ -370,6 +370,9 @@ This also works for the asciidoctor `:stem: latexmath`. For details see the [asc
 
 ### Mermaid
 
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15107) in
+GitLab 10.3.
+
 > If this is not rendered correctly, see
 https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/markdown.md#mermaid