Break upgrade guides into separate pages

[avantgardnerio]

Is your feature request related to a problem or challenge?

We are upgrading from DataFusion 45 -> 46, and it's been very helpful to have an upgrade guide! Thanks @alamb and whoever else is involved :)

However, it can be hard to navigate these guides, because they are all one page. For example, I see we had to make changes in our code involving Aggregate and WindowFunctions, but when I ctrl-F for relevant terms it scrolls me to random locations in the list of upgrades and I don't know if I'm on 46 or not still.

Describe the solution you'd like

Break the upgrade guides up into separate HTML pages so ctrl-F works again.

Describe alternatives you've considered

Saving as a PDF and cropping to just the 46 related pages so I can search. Come to think about it, that's exactly what I'm going to do right now.

Additional context

No response

[avantgardnerio]

n/m @dispanser directed me here: https://github.com/coralogix/arrow-datafusion/tree/v46/dev/changelog

[alamb]

This is probably a good first issue