YAML format should allow tracking applications using extensions (#152)

* YAML format should allow tracking applications using extensions

https://github.com/familysearch/GEDCOM-registries?tab=readme-ov-file#registering-extensions
says that "once an extension meets the Valuable, Absent, and Used
criteria then a standard tag/URI can be added...".

https://github.com/FamilySearch/GEDCOM/tree/main/attribute-event-requests
tracks what things are used by apps, but it's only for attributes and
events, not other extensions.

This PR proposes the ability to tracking "used by" in any YAML file
that describes an extension.

Signed-off-by: Dave Thaler <dthaler@armidalesoftware.com>

* Update _pages/yaml-file-format.md

* Update _pages/yaml-file-format.md

---------

Signed-off-by: Dave Thaler <dthaler@armidalesoftware.com>
Co-authored-by: Dave Thaler <dthaler@armidalesoftware.com>
Co-authored-by: Luther Tychonievich <tychonievich@gmail.com>

_pages/yaml-file-format.md

@@ -294,6 +294,17 @@ Their names may be changed a YAML file with a `lang` other than `en`.
 
     If `superstructures` is an empty `map`, then the structure is a record and must not appear in any other structure's `substructures`.
 
+-   <table><tbody>
+    <tr><th>Key</th><td><code>used by</code></td></tr>
+    <tr><th>Type</th><td><code>seq</code> of <code>str</code></td></tr>
+    <tr><th>Required by</th><td>—</td></tr>
+    <tr><th>Allowed by</th><td>all</td></tr>
+    </tbody></table>
+
+
+    A list of [`g7:HEAD-SOUR`](https://github.com/FamilySearch/GEDCOM-registries/blob/main/structure/standard/HEAD-SOUR.yaml)
+    values of applications known to use an extension tag for this concept.
+
 -   <table><tbody>
     <tr><th>Key</th><td><code>value of</code></td></tr>
     <tr><th>Type</th><td><code>seq</code> of URI</td></tr>