ENH: switch to the mystmd enginery for rendering #342

Job	Run time
ubuntu-latest with Python 3.13	1m 26s
macos-latest with Python 3.13	1m 12s
ubuntu-latest with Python 3.14 and developer versioned dependencies	2m 8s
ubuntu-latest with Python 3.10 and oldest versioned dependencies	1m 25s
windows-latest with Python 3.13	2m 1s
ubuntu-latest with Python 3.11 and latest released version of dependencies	1m 21s
ubuntu-latest with Python 3.12 and latest or pre-release version of dependencies	1m 30s
	11m 3s

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