Add interface schema and docs for sloth interface

We landed the [sloth interface library](https://github.com/canonical/charmlibs/tree/main/interfaces/sloth) in https://github.com/canonical/charmlibs/pull/321 without interface docs and schema in the currently standard format (they're folded into the library instead). This should be addressed in a follow-up PR.

We're planning some changes around the standard format, so let's tackle this afterwards as an early adopter of the new approach.

Start by reviewing the sloth interface library under interfaces/sloth and the standard interface schema and documentation format once the planned format changes land. Separate the schema and documentation from the library as required by that format; done means the sloth interface has both in the current standard form.

Written by the indexing model from the issue text.