doc: board: beagle: Add BeagleBone AI-64 documentation

Add base documentation for BeagleBone AI-64.

Signed-off-by: Nishanth Menon <nm@ti.com>
diff --git a/doc/board/beagle/index.rst b/doc/board/beagle/index.rst
index e6f9b74..9124546 100644
--- a/doc/board/beagle/index.rst
+++ b/doc/board/beagle/index.rst
@@ -11,3 +11,4 @@
    :maxdepth: 2
 
    am62x_beagleplay
+   j721e_beagleboneai64