Update sphinx dependency on `docmain` #6307

Job	Run time
Check tket library version	21s
Build and test (tket)	0s
Build and test pytket (ubuntu)	8m 33s
Build and test pytket (windows)	14m 52s
Build and test pytket (macos) (macos-13)	8m 40s
Build and test pytket (macos) (macos-15)	4m 35s
Build and test (tket)	0s
Check documentation build	0s
Check C++ code formatting	0s
Check pytket line and branch coverage	6s
Publish pytket coverage	0s
All build and test jobs successful (or skipped)	1s
	37m 8s

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