Add documentation outlining the workflow process for opening a SIG-NET-PEP or NPEP

[tssurya]

This was discussed in yesterday's policy-api meeting and we are going to do something similar to what gateway folks have: https://gateway-api.sigs.k8s.io/geps/overview/

[tssurya]

/assign @tssurya
I can do this as I attempt the first SIG-NET-PEP