WIP: Customise the sunpy-sphinx-theme #886

Job	Run time
sdist_verify	14s
tests / Load tox environments	5s
docs / Load tox environments	4s
allowed-fail-tests / Load tox environments	5s
docs / build_docs-notebooks (ubuntu-latest)	10m 59s
tests / py312 (ubuntu-latest)	5m 25s
tests / py311 (ubuntu-latest)	3m 39s
tests / py311-online (windows-latest)	7m 47s
tests / py310 (macos-latest)	8m 0s
tests / py310-oldestdeps (ubuntu-latest)	4m 23s
allowed-fail-tests / py311-devdeps (ubuntu-latest)	2m 55s
publish	0s
notify	0s
	43m 36s

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