Create a new document about support policies in readthedocs

* Imho support policies are important enpugh to give it more relevance
* Users should have a place to consult it to prepare and plan their migrations
* We have a note about window support for ghc versions used to build Cabal and cabal-install in a list o points inside[ contributing.md](https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#conventions)
* * `cabal install cabal-install` is a recommended way to upgrade cabal-install so that support would affect even newcomers
* It could be a generic document about support policies to be able to add other support windows (like which ghcs we support at runtime)

(Moreover i think contributing.md should be also in readthedocs, but it would be another issue)