Document `jhack scenario snapshot`

[dimaqq]

It's an awesome tool:

• get live deployments in an interesting state
• run the tool
• it produces Sceanario state and context that matches live
• continue developing much faster (local, unit, TDD)

Admittedly, jhack could use an update in this respect (scenario -> testing; imports; etc.), that could be part of this ticket.

Then we can also advertise it in our docs and/or charmhub.

[benhoyt]

Could put a link/explanation to this in the Relations how-to.