[ADF-3514] Added transclusions sections to doc files (#3756)

* [ADF-3514] Added transclusion sections and guide page

* [ADF-3514] Updated tables of contents where needed

* [ADF-3514] Updated index files

docs/user-guide/README.md

@@ -9,6 +9,7 @@ on an item below to see the corresponding guide page.
 -   [Form Extensibility and Customisation](extensibility.md)
 -   [Internationalization in ADF](internationalization.md)
 -   [Theming](theming.md)
+-   [Transclusion](transclusion.md)
 -   [Typography](typography.md)
 -   [Walkthrough - adding indicators to highlight information about a node](metadata-indicators.md)
 

docs/user-guide/transclusion.md

@@ -0,0 +1,41 @@
+---
+Added: v2.6.0
+---
+
+# Transclusion
+
+Several components in ADF make use of **transclusion**, which is the technique
+of incorporating user-supplied content in the body of a standard component. In
+most cases, this is used to make small customizations (for example, the various
+list components let you supply custom content to show when the list is empty).
+However, there are also a few "containers" whose entire content is set by the user
+with the container itself being mainly for convenient display and formatting
+(for example, the [Accordion component](../core/accordion.component.md)).
+
+You supply the content you want to transclude between the opening and closing tags of
+the main component. In a few cases, this content can be completely free-form as with
+the body section of the [Login component](../core/login.component.md):
+
+```html
+<adf-login ...>
+    <div>
+        <div>Your extra content</div>
+    </div>
+</adf-login>
+```
+
+More often, though, the main component makes use of one or more sub-components to add
+structure to the transclusion. For example, the [Login component](../core/login.component.md)
+also has sub-components for the header and footer regions in addition to the free-form
+content of the body:
+
+```html
+<adf-login ...>
+    <login-footer><ng-template>My custom HTML for the footer</ng-template></login-footer>
+</adf-login>
+```
+
+![Custom login footer example](../docassets/images/custom-footer.png)
+
+The doc pages for the components that use transclusion contain full details of all
+supported sub-components and their usage.