docs: add code migration guides for v1.3.1 and v1.4.0, update v1.4.0 migration guide #1513

Job	Run time
build	1m 31s
devshell-tests (nixos)	20s
devshell-tests (macos)	44s
upload-chain-specs	19s
deploy-argocd	0s
deploy-rustdoc	1m 39s
local-environment-tests	0s
local-environment-tests-post-merge	9m 49s
argocd-tests	0s
local-environment-tests-alert	0s
teardown-argocd	0s
pre-merge-checks-complete	0s
post-merge-actions-complete	3s
local-environment-tests-post-merge-alert	8s
	14m 33s

Provide feedback