Add a Sphinx extension to document cached properties of slots classes #1960

Job	Run time
Build & verify package	16s
Verify dev env	10s
Check types using supported type checkers	18s
Tests on pypy-3.10	25s
Run doctests & render changelog	15s
Tests & Mypy on 3.9	18s
Tests & Mypy on 3.12	1m 4s
Tests & Mypy on 3.11	1m 0s
Tests & Mypy on 3.10	1m 12s
Tests & Mypy on 3.13	1m 7s
Tests & Mypy on 3.14	1m 2s
required-checks-pass	5s
Combine & check coverage	0s
	7m 12s

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