This removes duplicated content from CONTRIBUTING.md and consolidate
everything under doc/development/contributing/.

This also fixes missing links.

Lastly, this moves the style guides from the "Implement design & UI
elements" page to a new "Style guides" page.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Better make it clearer under each section what the labels are
rather than listing a subset of them in the section title

A section was moved in f644b8c8 but this got lost.

As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/42891