UI: Add Registries section referencing the community widget registry

[smessie]

Part of #718.

This adds the specification-side reference to the community widget registry that is being set up in
w3c/shacl-resources (companion PR:
w3c/shacl-resources#10). No community widgets are defined here — this only puts
the extension point in place so that widgets can be added later, in that repository, without any change to
this specification.

Changes to shacl12-ui/index.html:

• Add the shuic namespace prefix (http://www.w3.org/ns/shacl-ui-community/) to the prefix table and
  JSON-LD context.
• Add a short pointer from the Built-in Widgets section to the new Registries section.
• Add a new informative Registries section with a Community Widget Registry subsection, describing
  where the registry lives, the shuic namespace, and that adding a widget needs no spec change. It also
  notes that other extension points (such as property roles) may follow the same pattern.

Note: the community namespace http://www.w3.org/ns/shacl-ui-community/ is pending confirmation and W3C
team allocation (see the discussion in #718).

• See this document rendered online here

[danielbeeke]

Looks good to me