doc(v3): add first draft of migration guide with links to it (#15839) #8999

Job	Run time
debug_job	2s
Save PR ref	3s
Windows-Bazel / bazel + msvc-2022 + opt + Core	3m 40s
Windows-CMake	0s
macOS-CMake	0s
macOS-Bazel	0s
Notify-Google-Chat	3s
	3m 48s

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