Documentation improvements for v5

[pauldendulk]

This issue is to keep track of what we should improve in our documentation before the release of v5.

Things that I have in mind right now:

• Start using the mkdocs version of our site: https://mapsui.com/v5/documentation2/
• Update to front page of the mkdocs version to contains the most important scenarios, so that the majority of users need nothing more than that page.
• Update the v5 upgrade guide.
• There are a number of issues with the 'docs-need-update' tag. Go through these and update what needs to be updated.
• The samples should be improved to function as proper documentation. More comments withing the samples. Better organization of topics, so that users can find what they are looking for when browsing through the samples.