gh-127833: Add links to token types to the lexical analysis intro

[encukou]

In the first part of the “lexical analysis” document, add links to token type documentation in the token module.

Add a section on the ENDMARKER token.

(I plan to send separate PR(s) for the rest of the document.)

• Issue: Align the grammar documentation with Python's actual grammar #127833

[AA-Turner]

I'm not keen on 'equivalent to', perhaps:

Suggested change

	token, but ``a b`` is two tokens. For another example, ``+a`` is two tokens,
	and is equivalent to ``+ a``.
	token, but ``a b`` is two tokens. However, ``+a`` and ``+ a`` both tokenize to the same output, as ``+a`` is not a valid token.