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>

- Also fix some markdown syntax.

fix design artifact label to be product discovery. Was done here https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20862 but not merged before this was moved.