Improve docs about security considerations

[m4dz]

As reported by a user, our documentation concerning security (SSL renewal, CouchDB password) are too thin, and we may add some extra infos to drive users through this cases: http://cozy.io/fr/host/install.html#consid-rations-sur-la-s-curit-avec-les-images-pr-install-es.

[audreytoskin]

The handwaving over security concerns in the documentation is also strange. I just read the "Play with the Data System" page (at src/documents/en/hack/getting-started/play-with-data-system.html.md).

you must be aware that we don't do security checks and data validation in the tutorial,
because it is not the point. If you want to know how to do it, please ask us on IRC or by email

It was probably a good idea to skip the security tutorial in the middle of the lesson about basic usage of the Data System. But I think security is important enough that it should be part of the official docs, not just something users have to ask about in IRC or email, where no one else can read it.

Telling users "If you're curious about security, send us an email" is silly. It should be "If you're curious about security, go this other page which talks about it in detail."