Clarify that you do not need to set the toc macro if you have the "On this page" widget activated for your site.
Overview
If merged, this MR will add a clarifying statement about not having to explicitly set the toc::[] macro on all pages for the "On this page" widget to show sectional navigation.
Background
In the Zulip chat^ I discovered that you do not need to set the toc::[] macro when using the "On this page" widget.
Because using this macro is such a reflex action for most Asciidoctor users, I wanted to be super explicit that you do not need to set this macro in the context of Antora.
It turns out that the Default UI has supported not setting the toc::[] macro for several years now.
Hopefully this addition will help people moving from Asciidoctor to Antora to understand how Antora handles things they were used to controlling manually.