Frontity Feature: Extensible

In the docs we have this feature of Frontity.

Is this extensibility refered to “packages”?
What was the purpose of this page linked in this section → https://github.com/frontity/docs/blob/a6a79476ae299fdebf52cf13ddeaca9ede179f24/docs/frontity-features/extensions.md ?
What is this “interface tools” refering to?