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

[LGLO]

Description

I'm using code migration guides, that were on separate branch. They were meant to be included in the release notes, I change the course, because apparently there was some duplication.

Another change is that v1.4.0 migration guide now assumes v1.3.1 as starting point.

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• New tests are added if needed and existing tests are updated.
• Relevant logging and metrics added
• CI passes. See note on CI.
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff

Note on CI

If your PR is from a fork, the necessary CI jobs won't trigger automatically for security reasons.
You will need to get someone with write privileges. Please contact IOG Partner Chains developers to do this
for you.