Add a major changes site in the documentation

[RobPasMue]

We should add a page in our hosted docs for the actions where we define the major changes between versions and the related modifications to be done.

[jorgepiloto]

Maybe we can start using Towncrier as an advance of #408.

[RobPasMue]

Hmm I doubt towncrier is the final solution. It won't be a pure changelog. I am also intending to add breaking changes guidelines on how to adapt to it. I will open a PR shortly.