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

[StanFromIreland]

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

(cherry picked from commit 2ff8608)

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

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