add page about abstract block style to nav; distinguish from abstract section

modules/sections/nav.adoc

@@ -14,6 +14,7 @@
 *** xref:dedication.adoc[]
 // & Epigraph
 *** xref:abstract.adoc[]
+*** xref:abstract-block.adoc[]
 *** xref:preface.adoc[]
 *** xref:parts.adoc[]
 **** xref:part-numbers-and-labels.adoc[]

modules/blocks/pages/abstract.adoc → modules/sections/pages/abstract-block.adoc

-= Abstract Block Style
+// REVIEW we may want to rethink where this page is located
+= Abstract (Block)
 
 An abstract is a concise overview of a document.
 The `abstract` block style can be placed on an open block or paragraph.

modules/sections/pages/abstract.adoc

-= Abstract
+= Abstract (Section)
 // anchor um: user-abstract
 
 An abstract is a concise overview of an article.