gh-135676: Simplify docs on lexing names (GH-140464)

This simplifies the Lexical Analysis section on Names (but keeps it technically correct) by putting all the info about non-ASCII characters in a separate (and very technical) section.

It uses a mental model where the parser doesn't handle Unicode complexity “immediately”, but:

- parses any non-ASCII character (outside strings/comments) as part of a name, since these can't (yet) be e.g. operators
- normalizes the name
- validates the name, using the xid_start/xid_continue sets


Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Blaise Pabon <blaise@gmail.com>
Co-authored-by: Micha Albert <info@micha.zone>
Co-authored-by: KeithTheEE <kmurrayis@gmail.com>