Add documentation for NamedNodeURIField

Adds documentation for the NamedNodeURIField form field type which was
missing from the forms-intro.html reference documentation and the
demo.html examples.

This field type prompts users for a URI and stores the value as an RDF
named node (resource) rather than a string literal. Unlike PhoneField
or EmailField, no URI prefix is added.

Closes #565

Author: melvincarvalho

docs/form-examples/demo.html

@@ -246,6 +246,25 @@ <h1>Form fields</h1>
             </td>
             <td class="output"></td>
           </tr>
+
+          <tr class="form-demo">
+            <td class="input">
+              <pre>
+# A field for entering a URI as a named node
+# (stores as RDF resource, not string literal)
+
+ex:form
+   a ui:NamedNodeURIField ;
+   ui:label "Homepage" ;
+   ui:property foaf:homepage .
+</pre
+              >
+            </td>
+            <td class="target">
+              <pre>ex:this foaf:homepage &lt;https://example.org/alice&gt; .</pre>
+            </td>
+            <td class="output"></td>
+          </tr>
         </tbody>
       </table>
     </div>

docs/forms-intro.html

@@ -396,6 +396,21 @@ <h3>Single Value fields - Text</h3>
 
     <h4 class="class">SingleLineTextField</h4>
 
+    <h4 class="class">NamedNodeURIField</h4>
+
+    <p>
+      A NamedNodeURIField is like a SingleLineTextField, except that the
+      value it generates is not a literal string but an RDF named node with
+      the given URI. Use this when users need to provide explicit URIs to things,
+      storing them as proper RDF resources rather than text values.
+    </p>
+
+    <p>
+      Unlike PhoneField or EmailField which add URI scheme prefixes (<tt>tel:</tt>
+      or <tt>mailto:</tt>), the NamedNodeURIField stores the URI exactly as
+      entered by the user.
+    </p>
+
     <h4 class="class">MultiLineTextField</h4>
 
     <h3>Complex fields</h3>