Update documentation for 2.0 changes

[stklcode]

This is a reminder for 2.0 release work.

We should update the documentation at https://statify.pluginkollektiv.org/documentation/

Probably incomplete list of points:

• Updated requirements
• Updated behavior (WP API migration, bot detction, ...)
• New hooks (e.g. "statify__skip_aggregation" when aggregation gets merged)
• Dropped IE11 support for the dashboard widget (see #272)