NEW Add migration task from gorriecoe/silverstripe-link

Description

Adds a migration task and guide for migrating from gorriecoe/silverstripe-link and gorriecoe/silverstripe-linkfield

Set up some links with gorriecoe/silverstripe-link
- be as creative as you want with it, hopefully the guide tells you how to deal with your scenario.
- Ideally have a range of relations
  - has_one
  - has_many (with custom sort column)
  - many_many (with many_many_extraFields)
  - many_many through (with extra $db fields)
  - many_many through, with polymorphic has_one on the "from" end of the join model
Use the upgrade guide. Where it says to composer require silverstripe/linkfield:^4 obviously you'll need to pull in this PR instead.

[x] The target branch is correct
- See picking the right version
[x] All commits are relevant to the purpose of the PR (e.g. no debug statements, unrelated refactoring, or arbitrary linting)
- Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
[x] The commit messages follow our commit message guidelines
[x] The PR follows our contribution guidelines
[x] Code changes follow our coding conventions
[x] This change is covered with tests (or tests aren't necessary for this change)
[x] Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
[ ] CI is green