Documentation on configuration of EDR Instances

[webb-ben]

Is your feature request related to a problem? Please describe.
I am looking to deploy an OGC API - Environmental Data Retrieval pygeoapi server that supports instances queries.

Describe the solution you'd like
Is there any documentation on how to implement an instances provider and what such a configuration would look like? If not, what would that sort of documentation look like?

Describe alternatives you've considered
I know https://github.com/geopython/pygeoapi/blob/master/pygeoapi/api/environmental_data_retrieval.py#L69 exists to provide API handling for instances, but I cannot quite derive how to implement and instances provider from this alone.

Additional context
Add any other context or screenshots about the feature request here.

[webb-ben]

@tomkralidis I am happy to produce documentation for this if you pointed me in the right direction.

[tomkralidis]

@webb-ben I've added some EDR plugin documentation as part of updating / fixing instances signatures in #2214.