Remove misleading migration docs/task for 4.0.0 release

[GuySartorelli]

There are migration docs and a task that were created for v3 that won't work for v4.

We're working on fixing these up for 4.1 but we missed the deadline for 4.0.

Acceptance criteria

• Remove migration task in 4.0
• Remove migration docs in 4.0
• Make sure to add back the migration index page for 4.x after merge-up

PRs

Reassign to Guy after merging so they can do the last acceptance criterion.

• https://github.com/silverstripe/silverstripe-linkfield/pull/259

[GuySartorelli]

Merge-up had conflicts. Resolved conflict and retained the docs that were up to date. All done now.