cli, docs: Add a docs command for doc generation

This took a lot longer than it looks to get right. It's not perfect, but
it now reliably generates documentation which we can put into gohugo.

cli/cli.go

@@ -123,6 +123,8 @@ type Args struct {
 
 	FirstbootCmd *FirstbootArgs `arg:"subcommand:firstboot" help:"run some tasks on first boot"`
 
+	DocsCmd *DocsGenerateArgs `arg:"subcommand:docs" help:"generate documentation"`
+
 	// This never runs, it gets preempted in the real main() function.
 	// XXX: Can we do it nicely with the new arg parser? can it ignore all args?
 	EtcdCmd *EtcdArgs `arg:"subcommand:etcd" help:"run standalone etcd"`
@@ -167,6 +169,10 @@ func (obj *Args) Run(ctx context.Context, data *cliUtil.Data) (bool, error) {
 		return cmd.Run(ctx, data)
 	}
 
+	if cmd := obj.DocsCmd; cmd != nil {
+		return cmd.Run(ctx, data)
+	}
+
 	// NOTE: we could return true, fmt.Errorf("...") if more than one did
 	return false, nil // nobody activated
 }