add chapter on Markdown and annotation

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+# Subsets of Markdown syntax structure
+# expressed in parsing expression grammar (PEG) notation.
+#
+# Copyright 2025, Jonas Smedegaard <dr@jones.dk>
+# SPDX-License-Identifier: GPL-3+
+
+# Document
+Markdown       <- MetaBlock* Block*
+
+# Block elements
+Block          <- Header / List / LinkDefinition / Paragraph
+Header         <- '#'+ SPACE* Words? SPACE* NEWLINE
+List           <- ([-*]+ / [[:digit:]]+ [).]) (SPACE+ (List / Words))?
+                  NEWLINE
+LinkDefinition <- '[' !'[]' PlainWords ']:' SPACE Uri NEWLINE
+                  SPACE SPACE '"' !'"' PlainWords '"' NEWLINE
+Paragraph      <- Words (SPACE? NEWLINE Words / HardBreak Words)*
+                  NEWLINE NEWLINE
+
+# Inline elements
+Words          <- StyledWords / LinkedWords / AnnotatedWords / PlainWords
+StyledWords    <- '*' Words '*' / '**' Words '**' / '_' Words '_'
+LinkedWords    <- '[' Words ']' '(' Uri ')'
+                / '[' Words ']' '[' ![\[\]] LinkDefinitionKeyphrase ']'
+                / '[' LinkDefinitionKeyphrase ']'
+AnnotatedWords <- '[' Words ']' '{' ![{}] PlainWords '}'
+PlainWords     <- PRINTABLES (SPACE PRINTABLES)*
+Uri            <- '<' ![<>] PRINTABLES? '>' / ![<>] PRINTABLES
+HardBreak      <- SPACE SPACE NEWLINE
+
+# Terminals
+PRINTABLES     <- [[:graph:]]+
+SPACE          <- ' '
+NEWLINE        <- '\r\n' / '\n' / '\r'