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>