docs(schemas): document the process for adding schemas and follow it for 3.1 series

[james-garner-canonical]

Description

We want to ensure that python-libjuju supports the latest Juju schemas, and that it's clear to python-libjuju maintainers and users what versions are supported. This PR adds documentation on how to do this (docs/CONTRIBUTING.md), and follows this process for the existing schemas, updating the 3.1 series (juju/client/SCHEMAS.md and juju/client/schemas-juju-*.json)

QA Steps

No changes to generated code are introduced in this PR. make client should not produce any changes to the repo state.

Test should all pass.

Notes & Discussion

Replaces:

• chore: add juju 3.1.{3,4,5,6,7,8,9,10} schemas, no code changes #1166

Implicitly depends on:

• fix: don't use lexical sorting for version numbers in codegen #1168
  Since we add a schema for 3.1.10 here.

[james-garner-canonical]

Docs need updating based on changed names/locations in

• chore: refactor client_facades and specified_facades shape #1155

I'll let you decide whether to merge this and then update the docs in #1155, or merge #1155 and then update this before merging, @dimaqq.

[dimaqq]

/merge