Reword the Lexical Analysis chapter of the docs

[encukou]

Documentation

Continuing with #127833, I'd like to reword and restructure the Lexical analysis section of the Reference documentation.

I think I found a style that works: each section will start with the most obvious things, expand toward a complete & precise description, with examples along the way, and end with the formal grammar. (Currently the grammar is usually listed first, and the author of the prose is tempted to assume that the reader has parsed and understood it.)

PRs merged for this before opening this issue:

• gh-127833: Add links to token types to the lexical analysis intro #131468
• gh-127833: lexical analysis: Improve section on Names #131474
• gh-127833: lexical analysis: Add backticks to BOM example #132407

Linked PRs

• gh-135676: lexical analysis: Improve section on Numeric literals #134850
• [3.14] gh-135676: lexical analysis: Improve section on Numeric literals (GH-134850) #135677