doc: add missing board/ti/index.rst

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