gh-141984: Reword the "yield expressions"` section

[encukou]

• Move discussion on the runtime behaviour of generators to builtins/stdtypes.rst.
• Reword and expand the section on yield expressions: start with an overview and common usage, then add details, end with formal grammar and examples.
• De-emphasize using yield to implement coroutines (the use case is now served by async/await.
• Copy an example from PEP-380 to keep the docs more self-contained
• In Simple Statements, merge yield_stmt into expression_stmt

This PR stops before the section on async generators (yield in async functions); that's left for the next PR.

• Issue: Reword the Expressions chapter of the docs #141984

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #35146121 | 📁 Comparing 8ae0306 against main (2588362)

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5 files changed · ± 5 modified

± Modified

• glossary.html
• builtins/stdtypes.html
• reference/datamodel.html
• reference/expressions.html
• reference/simple_stmts.html

[encukou]

(I'll fix the lint issues later; hopefully they won't block review.)