SES should be split into proposal and shim repositories

[erights]

I have now renamed tc39/proposal-frozen-realms to tc39/proposal-ses , as discussed at tc39. We should move SES documentation from https://github.com/Agoric/SES and elsewhere to here, update or remove stale documentation here, and rename https://github.com/Agoric/SES to emphasize that what remains there is just the ses-shim. We should parallel exactly the split we just did between proposal-realms and realms-shim.

(This issue is just https://github.com/Agoric/SES/issues/105 described from the other side.)