docs: Update Sphinx and myst-parser versions; resolve warnings #7462

Job	Run time
Linux tests (3.8)	10m 9s
Documentation	3m 13s
Linux tests (3.9)	9m 36s
Linux tests (3.11)	7m 45s
Linux tests (3.12)	9m 23s
Long tests on Python 3.10 (language parsers)	6m 37s
Long tests on Python 3.10 (scanners)	20m 30s
Tests that may fail due to network or HTML	4m 35s
Long tests on Python 3.10	36m 32s
Long tests on Python 3.10 (synchronous)	27m 5s
Windows long tests	1h 17m 52s
	3h 33m 17s

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