docs: add code migration guides for v1.3.1 and v1.4.0, update v1.4.0 migration guide #1512

Job	Run time
build	6m 39s
devshell-tests (nixos)	21s
devshell-tests (macos)	45s
local-environment-tests	6m 15s
deploy-argocd	19s
local-environment-tests-post-merge	0s
deploy-rustdoc	0s
upload-chain-specs	0s
local-environment-tests-post-merge-alert	0s
post-merge-actions-complete	0s
argocd-tests	2m 2s
teardown-argocd	19s
local-environment-tests-alert	11s
pre-merge-checks-complete	2s
	16m 53s

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