doc: add missing board/ti/index.rst

Commit 8baeeecbe305 ("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: 8baeeecbe305 ("doc: board: Move j721e document to doc/board/ti/ directory")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/board/ti/index.rst b/doc/board/ti/index.rst
new file mode 100644
index 0000000..c0da04b
--- /dev/null
+++ b/doc/board/ti/index.rst
@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Texas Instruments
+=================
+
+.. toctree::
+   :maxdepth: 2
+
+   j721e_evm