Add various improvements to the Events API migration guide

[skirtles-code]

The main changes here are:

1. Rename event hub to event bus.
2. Add some content discouraging the use of event buses. I've seen several examples on Discord where people used this page as proof that an event bus is an officially-endorsed strategy for handling component communication.
3. Add an explanation about passing events to the root component via createApp.

My bullet-points outlining alternatives to an event bus are somewhat terse. There's a huge amount of material that could be explored on that topic and explaining it in depth would need a whole page to itself. That said, I thought it was important to make some attempt at suggesting alternatives rather than just discouraging event buses.

I've moved the link to the migration build to the top of the 'Migration Strategy' section. I've split that section up into subsections and I didn't want the link to appear in a specific subsection.