Add migration material

[jku]

Add documentation and one time scripts for tuf-on-ci migration (#1247, see #929 for more context).

• Add migration plan
• Add playbooks for signers and maintainers
• add a short import script and a config file for the initial signing event

[jku]

Marking ready for review: I think this needs some other pair of eyeballs, mine are not useful here anymore

[kommendorkapten]

I too can be biased here, but I think the documentation is good.