Changes required for new Grace Hopper login environment

[bodgerer]

We now have a Grace superchip called gg001 that will eventually replace gh001 to provide the Grace Hopper login environment. Although not providing a Hopper GPU itself, it will be binary compatible and run the correct operating system.

This commit is an attempt to amend the documentation for when we bring it into service.

[ptheywood]

Looks good to me.

It might be worth changing the announcement in conf.py to highlight this change on all docs pages? Unless we want to change that sooner to state that the ghlogin node will change on a specific date.

I'll not merge this right now, but approve the PR so it can be merged when the change in ghlogin node is made.

[ptheywood]

Merging now as the ghlogin environment has now been updated to run on gg001 as of 2025-09-23.

ghlogin -A #######
srun: job 925745 queued and waiting for resources
srun: job 925745 has been allocated resources
[#########@gg001.bede ~] 

I'll follow this up with a separate change to the announcement (slighlty less faff to avoid conflicts and rebasing)