Integrations' documentation should be versioned

[silvanocerza]

As of now there's no way to see the documentation for an older integration version as we always overwrite with the latest changes.

We should give the possibility to users to read older versions of that documentation, so we should introduce versioning for integrations' docs too.

[masci]

This is not possible with Readme, so I don't think it can be part of the 2.1 milestone.