gh-145342: Reference the asyncio-guest example collection in the docs

The asyncio-guest project (the proof of concept this feature is based
on) has runnable guest-mode examples for Tkinter, Qt, GTK, pygame,
Win32 and Tornado hosts; point to it and to Trio's guest mode from the
guest mode docs and the module docstring.

Author: congzhangzh

Doc/library/asyncio-guest.rst

@@ -44,6 +44,17 @@ See :source:`Doc/includes/asyncio_guest_tkinter.py` for a complete Tkinter
 example that embeds asyncio inside ``tkinter.mainloop()`` using
 :func:`start_guest_run`.
 
+.. seealso::
+
+   The `asyncio-guest <https://github.com/congzhangzh/asyncio-guest>`__
+   project — the proof of concept this feature is based on — has runnable
+   examples for many more hosts: Tkinter, Qt (PyQt5/PySide6), GTK,
+   pygame, Win32 and Tornado.
+
+   `Trio's guest mode
+   <https://trio.readthedocs.io/en/stable/reference-lowlevel.html#using-guest-mode-to-run-trio-on-top-of-other-event-loops>`__,
+   which pioneered this approach.
+
 .. rubric:: API
 
 .. function:: start_guest_run(async_fn, *args, run_sync_soon_threadsafe, done_callback)

Lib/asyncio/guest.py

@@ -11,7 +11,12 @@
 A single "token" (semaphore permit) ping-pongs between the two threads,
 so exactly one of them touches the event loop at any moment.
 
-Inspired by Trio's guest mode (trio.lowlevel.start_guest_run).
+Inspired by Trio's guest mode (trio.lowlevel.start_guest_run).  The
+asyncio-guest project, which this implementation grew out of, has
+runnable examples for Tkinter, Qt, GTK, pygame, Win32 and Tornado
+hosts:
+
+    https://github.com/congzhangzh/asyncio-guest
 """
 
 __all__ = ('start_guest_run',)