Move the command index to the top of the CLI reference

[steverfrancis]

The https://docs.siderolabs.com/omni/reference/ page is fairly unusable, but it would be way better if the list of top level commands was at the top, not the bottom, of the doc.
Or even in a separate page above it.
I mean this: https://docs.siderolabs.com/omni/reference/cli#omnictl

Which is currently at the very bottom, which makes no sense from a users point of view.