Update prerequisites

Also, the `doc/source/user_guide/prerequisites_metoffice.rst` file needs updating to include setting up the mirrors at the MO. I would prefer not to include the specific instructions here (since they contain paths), but there will (soon!) be a link to the Git and GitHub documentation that we can add (which is visible to all members of the Met Office organisation on GitHub).

_Originally posted by @ehogan in https://github.com/MetOffice/CMEW/pull/361#discussion_r2965531222_

Open doc/source/user_guide/prerequisites_metoffice.rst and review its existing prerequisite guidance. Add the forthcoming Git and GitHub documentation link for setting up mirrors at the Met Office, without reproducing path-specific instructions; done means the file points readers to the accessible setup guidance.

Written by the indexing model from the issue text.