tools/net/ynl-gen-rst: Add sub-messages to generated docs

Add a section for sub-messages to the generated .rst files.

Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231215093720.18774-11-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>