Improve the contributing documentation

See merge request gitlab-org/gitlab-ce!22291

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>

`delete_all` doesn't support limit, so you'd need to subquery
that. And instead of subquerying with `where(id: query)`, it's better
to use an `INNER JOIN`. This method also works with MySQL, while
subquerying doesn't (without another layer of subquerying)

Reference:
https://stackoverflow.com/questions/17892762/mysql-this-version-of-mysql-doesnt-yet-support-limit-in-all-any-some-subqu/17892886#17892886

This enhances a few performance guideline titles in the development
README, making it easier to understand what topics they cover.

This is based on the discussion in
https://gitlab.com/gitlab-org/gitlab-ce/issues/49653 and the example
merge request found at
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20908. The idea
is fairly straightforward: we define a variety of abstractions, then
state which can reuse which. This should ultimately foster _better_ code
reuse, make it easier to debug problems, and make it harder to
accidentally introduce a performance regression.