Include examples for individual field values in schema reference page

[duncandewhurst]

In addition to the full worked example, it would be easier to refer implementers to examples for individual fields if these were included in the schema reference tables.

We'd need to look at how to rearrange the reference tables to fit the example in and some examples would be quite long, but we could use collapsing/expanding to take care of that, e.g.

[stevieflow]

@duncandewhurst looks interesting.

Tangential, but might be of interest: when @Bjwebb @caprenter and I did the IATI 2.01 documentation, we included the relevant block for the parent element in the (XML) schema, and then highlighted the particular sub-element that was relevant to the documentation page. It involved annotating an example XML file - which was then pulled in via rst. It worked - but was work!

Unfortunately, the latest version of IATI documentation has ripped all that out :) . The annotation in the example file, does remain.

[duncandewhurst]

Thanks @stevieflow, I was thinking along similar lines here, i.e. update the jsonschema sphinx directive to pull in the relevant bit of the example JSON file based on the JSON pointer, so I don't think we'd need to annotate the JSON file.

[jpmckinney]

There are a few places now in which JSON Schema is parsed for presentation. The Extension Explorer adds a few features beyond what sphinxcontrib-jsonschema offers – mainly, better reporting of types (e.g. "Period" with a link to its definition, instead of "object"). open-contracting/ocdskit#151 is about centralizing that logic, so that we don't need to add new features in multiple places going forward.