Add user guide docs for new SPECT, PET, and Linac models #2969

Job	Run time
build_opengate_wheel (ubuntu-latest, 3.9)	2m 34s
build_opengate_core_wheel (ubuntu-latest, 3.9)	12m 49s
build_opengate_core_wheel (ubuntu-latest, 3.10)	13m 59s
build_opengate_core_wheel (ubuntu-latest, 3.11)	13m 57s
build_opengate_core_novis_wheel	0s
build_opengate_core_wheel (ubuntu-latest, 3.12)	11m 56s
build_opengate_core_wheel (macos-13, 3.9)	9m 29s
build_opengate_core_wheel (macos-13, 3.10)	9m 29s
build_opengate_core_wheel (macos-13, 3.11)	12m 40s
build_opengate_core_wheel (macos-13, 3.12)	15m 3s
build_opengate_core_wheel (windows-latest, 3.9)	16m 6s
build_opengate_core_wheel (windows-latest, 3.10)	22m 51s
build_opengate_core_wheel (windows-latest, 3.11)	20m 11s
build_opengate_core_wheel (windows-latest, 3.12)	16m 50s
publish_wheel	1s
test_wheel	1s
publish_test	1s
	2h 57m 57s

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