Update MITgcm documentation with new build options file

[DaniJonesOcean]

Now that our eCSE project is over and the ARCHER2 build options file has been incorporated into the main MITgcm, I need to update the MITgcm ARCHER2 documentation. Please do feel free to assign this issue to me!

[kevinstratford]

In the meantime, I've removed the reference to the now non-existent options file.