Improve v5 to v6 migration documentation

There are some questions about the differences between nuts-nove v5 and v6. We need to improve the information so users can make better informed decisions.

We can use this page: https://nuts-node.readthedocs.io/en/latest/pages/deployment/migration.html

Questions we need to answer:

• What is the concept of a subject? (For a vendor, this will probably always be your customer)
• What is did:web and why do we now have 2 did methods?
• I use the did:nuts now as an identifier in my CRM, will this still work with the nuts v6 node after the migration and introduction of the did:web method?
• Can I use v5 use-cases with a v6 node?
• Can my v6 node communicate with a v5 node?
• If I want to use a v6 use-case on a v6 node, do I need to rewrite my v5 use-cases to v6 use-cases?
• What are my options for a SQL backend?
• How do I choose a DB backend? How much data will there be stored, hoe much read and wrote operations?
• What access-rights does the Nuts node need on the SQL server?
• How can I migrate the data from v5 to a SQL backend?
• What if the DB migration fails during startup?
• Can I stop using v5 data stores like Redis after updating to v6?
• Since all data is now stored in a DB, can I stop backing up the files in the data directory after updating to v6?
• What about this controller thing in the DID documents, how will this effect my current integration?
• Do I still need a vendor-did in my config file?
• If I don't upgrade to v6, how long will v5 be supported?
• If I use deprecated features in v6, when can I expect a v7 and how much time do I have to migrate?

Start with the migration page at nuts-node.readthedocs.io/en/latest/pages/deployment/migration.html and review the listed v5-to-v6 questions, especially the SQL backend, Redis, DID, and use-case topics. Update the migration documentation so each question has a clear answer about compatibility, configuration, migration, backups, support, and upgrade timing.