Added high-level Vaadin version comparison for migration

[samie]

Added a high-level comparison of Vaadin version features from Vaadin 8, Vaadin 14, Vaadin 23 and Vaadin 24 to Vaadin 25 to help us understand the scope of the migration effort.

[CLAassistant]

All committers have signed the CLA.

[jouni]

This page highlights the need for better table styles, which would allow for horizontal scrolling on narrow viewports.