Documentation: v1 to v2 migration guide.

cabal-install-3.0 changelog / release notes should point out to that part of documentation as the first things.

There will be a lot of confusion when 3.0 is released, and there are no migration guide explaining how `v2-build` or `v2-install` are different from `v1-build` or `v1-install`. The QuickStart section is not the right one, as it *doesn't explain the differences*.

Locate the cabal-install-3.0 changelog or release notes and the QuickStart section mentioned in the issue. Read the existing documentation for v1-build, v1-install, v2-build, and v2-install, then add or identify a migration guide that explains their differences and link to it from the release notes.

Written by the indexing model from the issue text.