docs: udpate defstruct docs with mention of field spec; add example;

[alexei]

Update defstruct docs to mention it's possible to use msgspec.field.

Addresses #580

[alexei]

@jcrist I suppose the documentation site is generated automatically?

[jcrist]

Thanks for the PR (and apologies for the delayed response)! I've pushed a small fixup, but otherwise this looks good to me.

I suppose the documentation site is generated automatically?

Yes, it's built and deployed as part of CI. If you want to build locally see https://github.com/jcrist/msgspec/blob/main/.github/CONTRIBUTING.md#documentation.

[alexei]

Thanks!