Document 0.7-rc.1 breaking changes

A bunch more breaking changes were added in the 0.7-rc.1 release. We should document them in the migration guide:

- The server context is gone
- Server functions return a different error type
- Server function data encodings are gone
- Middleware support is gone
- Multiple error support for the error boundary was removed
- The dioxus error type is no longer clone which means you can't call most resources like functions
- RSX context methods thrown from children to boundaries for errors and suspense are gone
- index.html support is gone

These changes also break a significant chunk of the doc examples that are checked (and likely more that are unchecked). We should go through each example and check them again before the release or address #537

Start with the migration guide for the 0.7-rc.1 release and review each listed breaking change. Then check the doc examples affected by these changes, including the checked examples mentioned in the issue. Done means the breaking changes are documented and affected examples are updated or confirmed to work before release.

Written by the indexing model from the issue text.