docs: fix readthedocs.io configuration
#798
| Job | Run time |
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| 1m 39s | |
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| 16m 11s | |
| 5m 43s | |
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| 8m 12s | |
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| 1m 48s | |
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| 1m 37s | |
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| 7m 49s | |
| 25m 58s | |
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| 8m 25s | |
| 28m 22s | |
| 8m 22s | |
| 9m 36s | |
| 0s | |
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| 3h 53m 9s |