CI: adding explicit sphinx config into RTD config #1095

Job	Run time
Py3.10 mindeps	4m 50s
Py3.11 mindeps	7m 11s
Py3.12 mindeps	7m 25s
Py3.11 no visualization + coverage	10m 4s
Py3.8 mindeps	3m 20s
Py3.9 noviz	5m 37s
Py3.9 all	5m 45s
Py3.11 dev	7m 40s
Py3.11 viz	8m 23s
Py3.11 all dev	7m 31s
Py3.8 mindeps and CASA	5m 11s
Py3.10 mindeps and CASA	6m 5s
Py3.11, noviz, dev	7m 28s
Py3.8 noviz (except CASA)	3m 46s
Py3.11 noviz (except CASA) MacOS X	4m 16s
Py3.11, noviz Windows	12m 28s
Py3.11, noviz, dev; Windows	12m 30s
Documentation	1m 21s
	2h 0m 51s

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