gh-141984: Move generator iterator reference out of syntax docs

[encukou]

The Expressions section of the docs contains reference documentation (that is, descriptions of all the methods) for Generator-iterators and Asynchronous generator-iterators.
These look out of place here, and interfere with explaining the syntax around yield.

There are two places that would make a better home for these: stdtypes in Library and datamodel in Reference. (These two have a lot of overlap, for what I think are mostly historical reasons, but fixing that is out of scope for this PR.)
I think stdtypes is the better place to put these.

I added an introduction to Generators here, which currently overlaps a bit with the Yield expressions in the grammar docs. I'll be reworking Yield expressions next.

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

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #33819163 | 📁 Comparing 02b643f against main (bfc16a7)

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21 files changed · ± 21 modified

± Modified

• glossary.html
• c-api/veryhigh.html
• howto/a-conceptual-overview-of-asyncio.html
• howto/functional.html
• library/asyncio-dev.html
• library/asyncio-eventloop.html
• library/collections.abc.html
• library/contextlib.html
• library/dis.html
• library/exceptions.html
• and 11 more...