docs: Reference document for the Service Factory

[lengau]

CRAFT-4508

Rendered: https://canonical-craft-application--1001.com.readthedocs.build/1001/reference/services/

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[medubelko]

Did you expect this for the method signatures?

[lengau]

@medubelko yes, that's the nature of the method signatures with so many overloads. I'm not sure there's a good way to remove it, and even if there were I'm not sure we should.

[medubelko]

I think this is fine for an index to start. If we can get more time behind our libraries, we can make the doc architecture a higher priority.