Update README for SIGs with Kernel and GRC links

[dcmiddle]

The SIGs README needs a couple updates
https://github.com/confidential-computing/governance/tree/main/SIGs

The Kernel SIG charter is in the governance repo but is not reflected in the README.
Please add it. Ideally add link for the agenda too.
Agenda - https://docs.google.com/document/d/18D820gyt2xe84Jy6dGXgrNThApoozcekDzvIlK2_bd4/edit?tab=t.0

In the README, the GRC has a link to the agenda but not to the charter committed to the repo.
https://github.com/confidential-computing/governance/blob/main/SIGs/GRC/Governance_Risk_and_Compliance_SIG_Charter.pdf

Consider also adding a link for all SIGs to the CCC Calendar. Or if we can point to elsewhere in the governance repo where that link already exists that might even be better (so when the calendar link changes we have fewer places to fix).
https://confidentialcomputing.io/get-involved/calendar/