Merge duplicated installation instructions

[stevapple]

Currently we're having two copies of installation instructions, in getting-started and download. This is not ideal and can be easily out of sync (Windows part is already out of sync now).

It would be better to provide single source of truth for that.

[alexandersandberg]

Hey @stevapple, we've made some improvements on this in our content improvements initiative which is now ready for review: #354

We would appreciate your feedback there! 🙂

[stevapple]

I think that's really neat!