Sort the sub-commands so that every time we execute the script it generates the exact same results.
This will case less merge conflicts if two PRs edit different json files.

also:
* make the script agnostic to where it is executed (more flexible).
* add documentation about commands.c and the json files in the readme.

Co-authored-by: Oran Agra <oran@redislabs.com>

With this rule, the script to generate commands.c from JSON runs whenever commands.o is built if any of commands/*.json are modified. Without such rule, it's easy to forget to run the script when updating the JSON files.

It's a follow-up on #9656 and #9951.