Define SIG proposal process

[dcmiddle]

While we have now created 3 SIGs with a 4th on its way, we have not documented the process.

For reference the project proposal process is here:
https://github.com/confidential-computing/governance/blob/main/project-submission-template.md
With an issue template
https://github.com/confidential-computing/governance/blob/main/.github/ISSUE_TEMPLATE/project-proposal-template.md

We could use similar governance documentation for SIG proposals.
Incomplete list of aspects...

• Create a charter template
• Document practice: use a public document like google doc or hackmd shared markdown to rough out the charter
• Once the charter is near final submit a PR for TAC review and eventual vote
  • https://github.com/confidential-computing/governance/tree/main/SIGs
• A TAC criteria for acceptance should be some informal resource commitment from TAC members