Add Feature LD viewer to examples

[rob-metalinkage]

Provide an optional configuration to use a Feature-LD viewer for examples.

Note that the viewer may have preconfigured graph closures for known applications - e.g. one pre-loaded with all the OGC ontologies and the Oceans Information Model components - so may need to be customised - also applications may have specialised viewers.

So a viewer link with a template URI, and a default pointing to an OGC hosted default perhaps?

Note that http: and https: deployments of viewer may be required if there is a fundamental limitation accessing http resources. The user coudll force the http in the config, but they would need to do this with explicit intention.

[rob-metalinkage]

Functionality now complete - need to have it deployed and we can manually link examples in the short term via MD text, but a template link in config automatically included would be better.