Provide a way of searching for usages of a LinkML construct

[cmungall]

Currently users have a few different ways of getting examples of usages of LinkML constructs.

The best place to start is the main documentation. However, space limitations mean that we prefer to show a limited number of examples so as not to overwhelm (but we likely could show a lot more!)

The URI of the element also leads to the main rendered docs, but this is currently short on examples..

We should make it easier to find usages:

1. Leverage the LinkML-registry to search schemas in the wild to see usages
2. Have docgen inline the examples folder that is a defacto standard (including for the metamodel itself)
3. The LinkML compliance tests have exhaustive (if a little concise) combinatorial examples, together with how they translate - we are currently working on making this output more transparent