Improve interface documentation

[Pen Gale]

Per the discussion in https://discourse.juju.is/t/interfaces-relations-and-how-to-help-casual-users-implement-them-in-juju/3700, the documentation for Juju interfaces is currently lacking.

This bug will be resolved when:

1) Someone without deep Juju knowledge can read the docs, and have a good idea how they might go about implementing an interface at a low level (e.g., without a framework).
2) We have documented, or linked to documentation on, the process for writing interfaces with the new Operator framework.

[Pen Gale (pengale)]

I believe that this fits under the documentation/tutorial work that we've added to the roadmap for this cycle. Targeted to the 3.0.0 milestone.

[Juan M. Tirado (tiradojm)]

Old issue. Reopen if needed.

[Launchpad Janitor (janitor)]

[Expired for Canonical Juju because there has been no activity for 60 days.]