Create proper upgrade guide

[sneridagh]

• Refactored the Content type to allow TypeScript to narrow this type automatically volto#8277
• Review https://volto.readthedocs.io/seven/upgrade-guide/index.html and determine how to organize.

[stevepiercy]

For reference, here's a link to the current collection of upgrade guides. I added it to the description.

https://volto.readthedocs.io/seven/upgrade-guide/index.html