API docs: $skiptoken and Dataset name conflict

[ktuite]

Closes getodk/central#580

$skiptoken was introduced in 2023.4 but not mentioned in the API docs, probably because there isn't much to say about it. It is part of the @odata.nextLink, which was already mentioned.

There might be more to add, though:

• $skip and $skiptoken can't be used together
• $skiptoken and $order can't be used together
• If you use $skip originally, you'll get a $skiptoken in the nextLink

I also added the note about Dataset name capitalization conflicts (e.g. "trees" vs. "Trees") and added a 409 conflict response, though I actually chose the response for the direct dataset name conflict.

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

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?

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

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

[matthew-white]

There might be more to add, though:

• $skip and $skiptoken can't be used together
• $skiptoken and $order can't be used together
• If you use $skip originally, you'll get a $skiptoken in the nextLink

I agree that it's not particularly useful to mention these things.