Expand the 'Extending' docs with an example. #110833

Job	Run time
Check for source changes	10s
Windows (free-threading)	0s
Windows	0s
Check if generated files are up to date	0s
macOS	0s
macOS (free-threading)	0s
Ubuntu	0s
Ubuntu (free-threading)	0s
Address sanitizer	0s
Thread sanitizer	0s
Thread sanitizer (free-threading)	0s
Ubuntu SSL tests with OpenSSL	0s
WASI	0s
CIFuzz	0s
Hypothesis tests on Ubuntu	0s
Docs / Docs (Oldest Sphinx)	1m 54s
Docs / Doctest	15m 25s
Docs / Docs	2m 7s
All required checks pass	2s
	19m 38s

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