Better discoverability of external API

[Kircheneer]

Environment

• DiffSync version: latest

Proposed Functionality

Add documentation around all public parts of the API. We are already generating documentation for a good amount of things from the docstrings, let's extend that to more parts of the codebase. For example, get_or_instantiate is a valuable feature that is currently only discoverable through the source code.

Use Case

Easier usage.