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/ti/k3.rst b/doc/board/ti/k3.rst
index f3bb664..83c46ac 100644
--- a/doc/board/ti/k3.rst
+++ b/doc/board/ti/k3.rst
@@ -36,6 +36,7 @@
    am64x_evm
    am65x_evm
    j7200_evm
+   ../beagle/j721e_beagleboneai64
    j721e_evm
    j721s2_evm