atom/popover demo improvements

[oegea]

We've been adding a new feature to the atom/popover component, to expose a method with which recalculate the popover position. (More info in this PR: https://github.com/SUI-Components/sui-components/pull/2036)

As a result of these changes, we have modified the readme.md file to document how to obtain this new function, and how to interact with it. However, @andresin87 told us that it could be possible to improve atom/popover demo to include an interactive demonstration of this new feature.