Some markdown headers needed tweaking to adhere
to standards, including blank lines above and below,
only one space after hash, first header should be
h1, and only one h1 per doc
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>