Documenting JSON queries

[c-martinez]

Because we use SPARQLTransformer, it is possible to express queries in JSON format. However, this functionality is not very well documented. We should improve the documentation to better communicate this functionality and provide some examples.

At the same time, it would be good to add tests to check JSON queries are properly handled and that JSON files which are NOT queries are safely ignored.

[c-martinez]

The second part of this

add tests to check JSON queries are properly handled and that JSON files which are NOT queries are safely ignored.

is covered by #487

[c-martinez]

An example query has been created in the queries repo