docs(material/sidenav): Document cdkFocusInitial for sidenav (#19564)

* docs(material/sidenav): Document cdkFocusInitial for sidenav
adds documentation about focusing inside the sidenav component
Fixes #19450

* cdkFocusInitial sentence rephrased

* sidenav autoFocus explained in details

* sidenav focus statement rephrased

* gramma error fixed

* rephrased autoFocus sentence

* Update sidenav.md

Co-authored-by: mmalerba <[email protected]>

Author: lukonik

src/material/sidenav/sidenav.md

@@ -209,6 +209,11 @@ Similarly, the `<mat-sidenav-content>` should be given a role based on what it c
 represents the primary content of the page, it may make sense to mark it `role="main"`. If no more
 specific role makes sense, `role="region"` is again a good fallback.
 
+#### Focus management
+The sidenav has the ability to capture focus. This behavior is turned on for the `push` and `over` modes and it is off for `side` mode. You can change its default behavior by the `autoFocus` input.
+
+By default the first tabbable element will recieve focus upon open. If you want a different element to be focused, you can set the `cdkFocusInitial` attribute on it.
+
 ### Troubleshooting
 
 #### Error: A drawer was already declared for 'position="..."'