Documentation: devlink: add missing toc entry for etas_es58x devlink doc

toc entry is missing for etas_es58x devlink doc and triggers this warning:

  Documentation/networking/devlink/etas_es58x.rst: WARNING: document isn't included in any toctree

Add the missing toc entry.

Fixes: 9f63f96a

 ("Documentation: devlink: add devlink documentation for the etas_es58x driver")
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/all/20221213051136.721887-1-mailhol.vincent@wanadoo.fr
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>