gh-127833: Reword and expand the Notation section (GH-134443)

Prepare the docs for using the notation used in the `python.gram`
file. If we want to sync the two, the meta-syntax should be the same.

Link the Full Grammar docs here; keep only a few extras.

Also, remove the distinction between lexical and syntactic rules,
except for whitespace handling.
With f- and t-strings, the line between the two is blurry.

Co-authored-by: Blaise Pabon <blaise@gmail.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>
Co-authored-by: Colin Marquardt <cmarqu42@gmail.com>