Add rule identifiers to notation chapter

We originally hadn't added rule identifiers to the notation chapter as
in some sense these aren't language rules.  However, as it turns out,
it's still useful to have these so as to be able to refer by name to
particular sections.  And anyway, since the language rules are defined
in terms of this notation, the notation rules are in a sense normative
at a meta level.

In doing this, we'll nest certain sections under the grammar section
that should be so nested.

Author: traviscross

src/notation.md

@@ -1,7 +1,11 @@
+r[notation]
 # Notation
 
+r[notation.grammar]
 ## Grammar
 
+r[notation.grammar.syntax]
+
 The following notations are used by the *Lexer* and *Syntax* grammar snippets:
 
 | Notation          | Examples                      | Meaning                                   |
@@ -26,7 +30,8 @@ The following notations are used by the *Lexer* and *Syntax* grammar snippets:
 
 Sequences have a higher precedence than `|` alternation.
 
-## String table productions
+r[notation.grammar.string-tables]
+### String table productions
 
 Some rules in the grammar — notably [unary operators], [binary
 operators], and [keywords] — are given in a simplified form: as a listing
@@ -40,13 +45,15 @@ When such a string in `monospace` font occurs inside the grammar,
 it is an implicit reference to a single member of such a string table
 production. See [tokens] for more information.
 
-## Grammar visualizations
+r[notation.grammar.visualizations]
+### Grammar visualizations
 
 Below each grammar block is a button to toggle the display of a [syntax diagram]. A square element is a non-terminal rule, and a rounded rectangle is a terminal.
 
 [syntax diagram]: https://en.wikipedia.org/wiki/Syntax_diagram
 
-## Common productions
+r[notation.grammar.common]
+### Common productions
 
 The following are common definitions used in the grammar.