Add pytket extensions index page to pytket API docs #4588

Job	Run time
Check tket library version	22s
Build and test pytket (macos) (macos-12)	14m 39s
Build and test pytket (ubuntu)	13m 29s
Build and test pytket (windows)	24m 3s
Build and test pytket (macos) (macos-13-xlarge)	3m 35s
Build and test (tket)	0s
Build and test (tket) (ubuntu-22.04)	3s
Build and test (tket)	0s
Check C++ code formatting	0s
Build and test (tket) (macos-12)	3s
Check documentation build	0s
Build and test (tket) (windows-2022)	1s
Build and test (tket) (macos-13-xlarge)	1s
Publish pytket coverage	0s
Check pytket line and branch coverage	0s
	56m 16s

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