The Documentation Release Process

Hi guys!

So as Symfony's release process has gotten more and more formal, the documentation team (primarily due to some really great community members) has also been updating branches as needed to manage our lifecycle.

This PR is to formalize that process so that we know exactly what do do at what time. There are 2 goals:

1) Make a guide that is really easy to follow - we know what to do exactly when
2) Make a guide that covers all of the changes that need to be made

This is not exactly the process we've followed in the past, and you may have some really great improvements, which I'd love to hear. Two new PR's (#2688 and #2690) both deal with these issues, so I think it's a good time to formalize things.

Thanks!