This adds a small line to the MR workflow to clarify that new source
installation requirements have to be added when they are introduced,
instead of release managers having to add them.
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>
Yorick Peterse