[FEATURE] Add a boilerplate 4 -> 5 mirgation guide

[dingo-d]

We were discussing in the slack how to migrate from old to a new version and maybe that could be added as a chapter in the docs, not overly detailed, just some basic steps (list of things to do).

Could be useful if we want to move older projects to new ones, but also if anybody using the older version would like to upgrade.

[iruzevic]

done