Update links and remove one in General Guidelines.
This updates a few links at https://about.gitlab.com/handbook/general-guidelines/:
- A few links are now relative to the web root, so they should now work properly in staging and testing contexts.
- A forwarded link now leads directly to the new URL.
- The "contract" link is no more. The text remains, but it is now unlinked. The relevant content now exists in a different location, but neither that nor the Contracts page actually seems relevant to the link context.