Add example content to OData /datasets/{name}.svc 200 response

[spwoodcock]

The problem this PR solves

The Service Document endpoint for datasets was missing an example 200 response:
https://docs.getodk.org/central-api-odata-endpoints/#id1

What was done to fix it

• Previously we had: content: {}.
• Updated to include actual content.

What has been done to verify that this works as intended?

Loaded the yaml into https://editor.swagger.io/

Why is this the best possible solution? Were any other approaches considered?

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

• I assume this will be copied to getodk/docs, so is user facing.

Does this change require updates to the API documentation? If so, please update docs/api.md as part of this PR.

• I think this PR template needs an update to reference docs/api.yaml ! 😄

Before submitting this PR, please make sure you have:

• run make test and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced

[lognaturel]

Looks good, thank you!

Do you have a use for the service document? We have so far assumed it's only of value to clients that already use the OData standard (Excel, etc). It could be somewhat useful for arbitrary clients using submissions because of repeats but it feels less clear to me for entities.

[spwoodcock]

No I don't use the service document, I just saw this when I was going through the docs 😄

I only use the data endpoint with filter and select params.

[matthew-white]

I think this PR template needs an update to reference docs/api.yaml ! 😄

Haha good catch! I've opened #1124 to make that change. Thanks for making a note of that.