docs: group the global flags and make them appear on command and flags pages

This adds an additional parameter to the creation of each flag. This
specifies one or more flag groups. This **must** be set for global
flags and **must not** be set for local flags.

This causes flags.md to be built with sections to aid comprehension
and it causes the documentation pages for each command (and the
`--help`) to be built showing the flags groups as specified in the
`groups` annotation on the command.

See: https://forum.rclone.org/t/make-docs-for-mortals-not-only-rclone-gurus/39476/

cmd/rmdir/rmdir.go

@@ -24,6 +24,9 @@ with option ` + "`--rmdirs`" + `) to do that.
 
 To delete a path and any objects in it, use [purge](/commands/rclone_purge/) command.
 `,
+	Annotations: map[string]string{
+		"groups": "Important",
+	},
 	Run: func(command *cobra.Command, args []string) {
 		cmd.CheckArgs(1, 1, command, args)
 		fdst := cmd.NewFsDir(args)