docs(material/sidenav): Document cdkFocusInitial for sidenav (#19564)
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 <7b2cb79a308d97ccf3cafbe982a2dc7a1de64584@google.com>
mmalerba/material2,josephperrott/material2,josephperrott/material2,angular/components,andrewseguin/material2,andrewseguin/material2,andrewseguin/material2,mmalerba/material2,mmalerba/material2,crisbeto/material2,angular/components,andrewseguin/material2,mmalerba/material2,andrewseguin/material2,DevVersion/material2,josephperrott/material2,mmalerba/material2,DevVersion/material2,crisbeto/material2,crisbeto/material2,DevVersion/material2,crisbeto/material2,josephperrott/material2,angular/components,josephperrott/material2,DevVersion/material2,angular/components,DevVersion/material2