docs: Expand processor/provider docs

[lucyleeow]

Expand the processor/provider docs in 'getting started'. In particular how the store iterates over processors/providers.

Hopefully it is correct 😬

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (29210cf) 100.00% compared to head (ad192d7) 100.00%.
Report is 4 commits behind head on main.

Additional details and impacted files

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[codspeed-hq]

CodSpeed Performance Report

Merging #98 will not alter performance

_{Comparing lucyleeow:doc_inj (ad192d7) with main (29210cf)}

Summary

✅ 5 untouched benchmarks

[tlambert03]

if we're going to add this, then the return type annotation should be changed to str | None. But I also feel like it's a bit contrived here... since Thing always has an attribute name... so it's not a great example of a processor that may or may not return a value (like, for example napari.get_current_viewer). So, if you think an example of this is needed (in addition to the text above in the "multiple providers" note) then let's add a new section with a new provider

[lucyleeow]

Yeah examples are hard. I don't think it's needed, the doc should be enough

[tlambert03]

Thanks @lucyleeow! Yep everything is pretty much correct! I made a couple slight wording changes and elaborations. One more comment left to discuss then lets merge.

[lucyleeow]

Oh one more thing I wanted to mention is that if a default value is provided, no provider will attempt to inject. It seems obvious but e.g. if you have Optional[sometype] = None, injection wouldn't be done.

[tlambert03]

yep! Good thing to add

[tlambert03]

(if you ever run into a case where you need/want to be able to override default values with injected values, that would be a reasonable feature request, probably added as a parameter to inject)

[lucyleeow]

Thanks for making the changes! I didn't have time to add a note on default values so did it here: #99