v3 migration guide for 'extension' removed methods

Fixes asciidoctor#1203

CHANGELOG.adoc

@@ -37,7 +37,9 @@ Improvement::
 * Return Document AST when using convert or convertFile with appropriate options (#1171) (@abelsromero)
 * Expose Links in the catalog (#1183) (@abelsromero)
 * BREAKING: Remove deprecated methods in Options, OptionsBuilder, Attributes & AttributesBuilder (#1199) (@abelsromero)
-* BREAKING: Remove deprecated Asciidoctor interface (#1201) (@abelsromero)
+* BREAKING: Remove deprecated methods from Asciidoctor interface (#1201) (@abelsromero)
+* BREAKING: Remove deprecated methods from Document interface  (#1202) (@abelsromero)
+* BREAKING: Remove deprecated methods from extension package (#1203) (@abelsromero)
 
 Bug Fixes::
 

docs/modules/guides/partials/removal-of-deprecated-constants-in-BlockProcessor.adoc

@@ -0,0 +1,38 @@
+== Removal of deprecated constants from `org.asciidoctor.extension.BlockProcessor`
+
+All constants in `org.asciidoctor.extension.BlockProcessor` class that were marked as `@Deprecated` have been removed.
+
+You can find the new values in `org.asciidoctor.extension.Contexts` interface.
+See the table below for the equivalencies.
+
+|===
+|Deprecated value |Contexts value
+
+|CONTEXTS
+|KEY
+
+|CONTEXT_OPEN
+|OPEN
+
+|CONTEXT_EXAMPLE
+|EXAMPLE
+
+|CONTEXT_SIDEBAR
+|SIDEBAR
+
+|CONTEXT_LITERAL
+|LITERAL
+
+|CONTEXT_LISTING
+|LISTING
+
+|CONTEXT_QUOTE
+|QUOTE
+
+|CONTEXT_PASS
+|PASS
+
+|CONTEXT_PARAGRAPH
+|PARAGRAPH
+
+|===

docs/modules/guides/partials/removal-of-deprecated-methods-in-extension-package.adoc

@@ -0,0 +1,20 @@
+== Removal of deprecated methods in `org.asciidoctor.extension` package
+
+Several methods under `org.asciidoctor.extension that were marked as `@Deprecated` have been removed.
+The new methods align better with Java naming patterns and are easily identifiable.
+
+[,java]
+.Removed deprecated methods
+----
+PreprocessorReader::push_include
+Reader::getLineno
+Reader::lines
+----
+
+[,java]
+.Final methods
+----
+PreprocessorReader::pushInclude
+Reader::getLineNumber
+Reader::getLines
+----