gh-135676: Simplify docs on lexing names

[encukou]

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:

1. parses any non-ASCII character (outside strings/comments) as part of a name, since these can't (yet) be e.g. operators
2. normalizes the name
3. validates the name, using the id_start/id_continue sets (referred to in previous sections as “letter-like” and “number-like” characters, with a link to the details)

This also means we don't need xid_start/xid_continue to define the behaviour :)

• Issue: Reword the Lexical Analysis chapter of the docs #135676

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📚 Documentation preview 📚: https://cpython-previews--140464.org.readthedocs.build/

[willingc]

Outstanding document @encukou. I had one small suggestion to be a bit more explicit on the normalization example with number.

[willingc]

It would be helpful to add an explicit comment that the normalized form of nᵘₘᵇₑʳis number.

[encukou]

Does this look good?

[encukou]

There was an insightful conversation in #140269. I'll update this PR to make things even clearer.

[willingc]

Thanks @encukou