docs: Add documentation for HeadingPlugin and TOCPlugin #1142

Job	Run time
coverage (3.10, dj42_cms311.txt, ubuntu-latest)	1m 4s
coverage (3.10, dj42_cms41.txt, ubuntu-latest)	1m 9s
coverage (3.10, dj50_cms41.txt, ubuntu-latest)	1m 15s
coverage (3.10, dj51_cms41.txt, ubuntu-latest)	1m 11s
coverage (3.11, dj42_cms311.txt, ubuntu-latest)	1m 1s
coverage (3.11, dj42_cms41.txt, ubuntu-latest)	1m 2s
coverage (3.11, dj50_cms41.txt, ubuntu-latest)	24s
coverage (3.11, dj51_cms41.txt, ubuntu-latest)	28s
coverage (3.12, dj42_cms311.txt, ubuntu-latest)	24s
coverage (3.12, dj42_cms41.txt, ubuntu-latest)	26s
coverage (3.12, dj50_cms41.txt, ubuntu-latest)	25s
coverage (3.12, dj51_cms41.txt, ubuntu-latest)	24s
	9m 13s

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