Add docs for rotating manually uploaded certs.
Add docs for rotating manually uploaded certs.

This process is somewhat gnarly due to limitations in terraform and the
way we've had to split things up.

Having said that, it's still valuable to have this documented until we
can come up with another approach (possibly not using terraform for this
specific bit).

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf