Fix Rake tasks doc README: add top level h1

and link to missing to features.md.

doc/raketasks/README.md

+# Rake tasks
+
 - [Backup restore](backup_restore.md)
 - [Cleanup](cleanup.md)
+- [Features](features.md)
 - [Maintenance](maintenance.md) and self-checks
 - [User management](user_management.md)
 - [Web hooks](web_hooks.md)