Docs: Generate Mimirtool documentation similarly to the config file documentation

Is your documentation request related to a feature? If so, which one?

This is related to the mimirtool documentation hosted at: https://grafana.com/docs/mimir/latest/operators-guide/tools/mimirtool/

Describe the solution that you’d like or the expected outcome

This document is hand-written and has fallen out of sync with the capabilities of the tool. Most command-line flags are undocumented. If this documentation was generated from the flags defined in cmd/mimirtool's different command files (located in pkg/mimirtool/commands), it would be possible to keep it up to date automatically. This could perhaps be done with a special script called by go generate.