Add migration doc for `source.dynamic`. Fixes: #4268 #11964

Job	Run time
build_opam (4.14.x)	4m 46s
build_details	5s
build_js	2m 14s
build_opam (5.2.x)	4m 42s
tree_sitter_parse	47s
build_no_depopts	4m 20s
build_win32 (x64)	4m 10s
build_posix (debian_trixie, amd64)	4m 20s
lezer_parse	52s
run_tests (@citest)	4m 18s
build_posix (debian_trixie, arm64)	3m 54s
update_doc	4m 21s
build_posix (debian_bookworm, amd64)	4m 19s
run_tests (@doctest)	4m 18s
run_tests (@mediatest)	4m 19s
build_posix (debian_bookworm, arm64)	3m 56s
build_posix (ubuntu_oracular, amd64)	4m 18s
build_posix (ubuntu_oracular, arm64)	3m 49s
build_posix (ubuntu_noble, amd64)	4m 19s
build_posix (ubuntu_noble, arm64)	3m 54s
build_posix (alpine, amd64)	4m 19s
build_posix (alpine, arm64)	4m 0s
fetch_s3_artifacts	1s
build_docker_alpine	1s
build_docker_minimal	1s
build_docker	1s
build_docker_alpine_minimal	1s
build_docker_release	1s
build_docker_release-minimal	1s
update_release	1s
	1h 20m 28s

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