Move external plugins chapter in a plugins section

doc/src/guide/book.asciidoc

@@ -28,8 +28,6 @@ include::escripts.asciidoc[Escripts]
 
 include::compat.asciidoc[Compatibility with other build tools]
 
-include::external_plugins.asciidoc[External plugins]
-
 = Documentation
 
 include::asciidoc.asciidoc[Asciidoc documentation]
@@ -54,6 +52,10 @@ include::dialyzer.asciidoc[Dialyzer]
 
 include::xref.asciidoc[Xref]
 
+= Third-party plugins
+
+include::external_plugins.asciidoc[External plugins]
+
 = About Erlang.mk
 
 include::why.asciidoc[Why erlang.mk?]