Commands must be sorted

[fnando]

Right now, commands aren't sorted, so they're listed out of order on both the overview list, as well as the main content.

See https://github.com/stellar/stellar-cli/blob/806cb398c3b9887e5e67f3b63016ec4cb2132636/FULL_HELP_DOCS.md for an example.

[crazyscot]

I would like them to appear in the same order that clap outputs them, respecting display_order() and grouped by help_heading(). Perhaps alphabetical sorting could be an option?