Docs: Add a few pointers to help new devs getting started.

Separate into User/Ops/Dev documentation.

Update PaymentServer to include additional metrics

Update PaymentServer to include new metrics publishing

Update PaymentServer to enable charge metrics

Upgrade Tahoe-LAFS on the storage servers

Re-importing the path probably breaks other stuff.  Anyway, we only have one
overlay now so this short/simpler thing works.

Rely on upstream package to supply typing and challenge-bypass-ristretto as
necessary.  Simplify our expression (approximately pointfree style) so we
don't have to know about any other arguments that might be passed or accepted.

Bump to branch that fixes the Copyable/ShareStat issue

Also adjust the system tests to account for the new authorization requirements.