Make edit_url: ~ apply to local file system urls.

The documentation https://docs.antora.org/antora/2.3/playbook/content-edit-url/ strongly implies that setting edit_url: ~ in the playbook will remove the 'edit this page' but it only does so for remote sources. This results in confusion (see todays Antora/users Gitter channel) and inconsistent behavior between author-mode and production.

Alternatively, the documentation should make it really really clear what the actual behavior is.