migrate documentation for the AsciiDoc language

docs/asciidoc/modules/blocks/pages/horizontal-rule.adoc

+= Horizontal Rule
+
+A horizontal rule is created by entering three single quotation marks (`++'''++`) on a line.
+
+.Horizontal rule syntax
+[source#ex-rule]
+----
+'''
+----
+
+The result of <<ex-rule>> is displayed below.
+
+'''
+
+== Markdown-style horizontal rules
+
+Asciidoctor recognizes Markdown horizontal rules.
+The motivation here is to ease migration of Markdown documents to AsciiDoc documents.
+
+To avoid conflicts with AsciiDoc's block delimiter syntax, only 3 repeating characters (`-` or `+*+`) are recognized.
+As with Markdown, whitespace between the characters is optional.
+
+.Markdown-style horizontal rule syntax
+----
+---
+
+- - -
+
+***
+
+* * *
+----
+
+A macro definition for the Markdown horizontal rules is included in the AsciiDoc compatibility file so they can be recognized by the `asciidoc` command as well.

docs/asciidoc/modules/blocks/pages/masquerade.adoc

+= Masquerading Blocks
+
+Some blocks can masquerade as other blocks, a feature which is controlled by the block style attribute.

docs/asciidoc/modules/blocks/pages/nest.adoc

+= Nesting Blocks
+
+You can nest blocks inside each other.
+To nest blocks of the same type, add a pair of extra symbols to the delimiter.
+For example:
+
+----
+====
+This is an example
+======
+This is an example inside an example
+======
+====
+----