Add subcommand that will generate example YAML that will parsable from Go structs

[bwplotka]

Similar to https://github.com/thanos-io/thanos/blob/main/scripts/cfggen/main.go

Acceptance Criteria:

• User can pass such tool in mdox-exec feature (separate subcommand?)
• Will generate example YAML (or whatever format is parsed to Go structs)
• Allows passing example defaults
• Preserve comments
• Optional: Tests if generated yaml can be actually parsable and returns err if not?