Add documentation for teams and projects

[gtsiolis]

Problem to solve

The upcoming Teams & Projects feature does not have any documentation at the moment. In addition, there's also some existing documentation about the current Teams feature, see /docs/teams which could cause some confusion to the users.

Proposal

Let's add some documentation around teams and projects as well as add a deprecation notice for the current Teams feature, following the changes in #5121.

[jldec]

Closing this - since we have docs now and even a Beta notice :)

[gtsiolis]

Thanks for coming back to this, @jldec! Missed closing this as the initial PR in #842 was replaced in https://github.com/gitpod-io/website/pull/842#issuecomment-944415830.