For me, annotating the reading position of a long document in the frontmatter and/or localstorage makes sense.
On the other hand, viewing (and, if necessary, annotating) paragraph by paragraph seems complicated to achieve for minimal gain.
But why not … within a widget. On this subject, the use of a resizable and movable frame (see Floating SidePanel/Widgets (Example: Document Explorer, Treeview, “OrbitCal” Calendar) - #19 by Mr.Red) might be useful.