Add nanopublication templates and conformance shapes docs

[VirginiaBalseiro]

This PR adds documentation and artifacts for the Nanoarguments templates and conformance shapes:

• templates.html: docs for the four published templates (discourse contribution, evidence finding, question, annotation), each linked to its nanopublication
• shapes.html: docs for the conformance shapes and how to run them
• shapes/conformance.shacl.ttl: the SHACL conformance shapes for Nanoarguments nanopublications
• notebooks/validation.ipynb: a runnable example that validates a nanopublication against the shapes (rdflib + pySHACL)