gh-116666: Add "token" glossary term (GH-130888)

Add glossary entry for `token`, and link to it.
Avoid talking about tokens in the SyntaxError intro (errors.rst); at this point
tokenization is too much of a technical detail. (Even to an advanced reader,
the fact that a *single* token is highlighted isn't too relevant. Also, we don't
need to guarantee that it's a single token.)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Doc/reference/lexical_analysis.rst

@@ -8,8 +8,9 @@ Lexical analysis
 .. index:: lexical analysis, parser, token
 
 A Python program is read by a *parser*.  Input to the parser is a stream of
-*tokens*, generated by the *lexical analyzer*.  This chapter describes how the
-lexical analyzer breaks a file into tokens.
+:term:`tokens <token>`, generated by the *lexical analyzer* (also known as
+the *tokenizer*).
+This chapter describes how the lexical analyzer breaks a file into tokens.
 
 Python reads program text as Unicode code points; the encoding of a source file
 can be given by an encoding declaration and defaults to UTF-8, see :pep:`3120`