doc: add missing board/ti/index.rst

Commit 8baeeecbe3 ("doc: board: Move j721e document to doc/board/ti/
directory") introduced 'make htmldocs' build errors due to a missing
file board/ti/index.rst.

Fixes: 8baeeecbe3 ("doc: board: Move j721e document to doc/board/ti/ directory")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/board/ti/index.rst

@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Texas Instruments
+=================
+
+.. toctree::
+   :maxdepth: 2
+
+   j721e_evm