exporters/makdown_singlepage: add ⬚ symbol for entries which have no recent commits

[nodiscc]

Example:

• sovereign ⬚ - Set of Ansible playbooks to build and maintain your own private cloud: email, calendar, contacts, file sync, IRC bouncer, VPN, and more. GPL-3.0 YAML/Other

And add it to the list of anti-features:

• ⬚ - The project is no longer actively maintained