play: add boot to demo list, but not crazy foldy menu thingy for now

boards/beagleplay/02-quick-start.rst

@@ -96,3 +96,5 @@ Demos and Tutorials
 * :ref:`beagleplay-oldi`
 * :ref:`beagleplay-csi`
 * :ref:`beagleplay-zephyr-development`
+* :ref:`play-kernel-development`
+* :ref:`play-understanding-boot`

boards/beagleplay/05-demos.rst

@@ -113,13 +113,13 @@ Demos and tutorials
     +++
     .. admonition:: Complexity level
 
-        intermediate
+        advanced
 
 .. card::
     :link: greybus-host
     :link-type: ref
 
-    **Bealgplay Greybus Host**
+    **BeaglePlay Greybus Host**
     +++
     .. admonition:: Complexity level