/docs doesn't redirect anywhere

[PragTob]

https://www.matestack.org/docs 404s but I believe it should rather be the main/index page for the docs.

For the time being it can/could redirect to install but in the long run I think it'd be better if it was an introductory summarizing what matestack is and maybe providing a high level outline of the documentation.

[pascalwengerter]

@jonasjabari I think this one can be closed!