Clarify optionality of type annotations for writing a plugin

[vreuter]

This would close #365 by making more explicit the optionality of type annotations when writing a plugin, and adding a general note about their utility in the doc.

[lucyleeow]

I would move this up to the 'The LayerData tuple' section even. Something like:

This type shows up often in plugins and is explained here to help plugin developers write
their functions. These annotations are optional except in the case of
[`magicgui` function widgets](magicgui) ...

The syntax at the end adds a link to the relevant widget section of the docs.

I've checked with Draga and there are no other places annotation is required for npe2, and I think it is not ever required for npe1 but I will confirm this! Edit - yeah I think never req for npe1.

[vreuter]

The syntax at the end adds a link to the relevant widget section of the docs.

Thanks @lucyleeow , I'd wanted to add this but wasn't quickly sure of exactly the right link reference

[vreuter]

@lucyleeow I took (as I understood it -- hopefully correctly) your suggestion about placement within the doc for this new text, and your idea to link to the magicgui section.

[lucyleeow]

Could you also add strings (or imports if you feel this is better) to the reader plugin code snippet too? Thank you!

[vreuter]

Could you also add strings (or imports if you feel this is better) to the reader plugin code snippet too? Thank you!

Yeah not 100% sure I see the right edit, but I gave a shot here: napari/npe2#340

[lucyleeow]

Ah yes I forgot our docs are spread out across 3 repos and are complicated 🤦 . I'll take a look!

[lucyleeow]

Thank you!

[lucyleeow]

Looks good, thank you!