tools/net/ynl-gen-rst: Sort the index of generated netlink specs

The index of netlink specs was being generated unsorted. Sort the output
before generating the index entries.

Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231215093720.18774-12-donald.hunter@gmail.com


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