Installation guide improvements (especially for Windows) (swiftlang#385)

* Categorize Windows installation options

* Fix typo: Addtional → Additional

* Minor fixes

* Add back the missing installer guide

* Add back instructions about developer mode

* Remove duplicated contents from `/download`

* Minor fixes

* Avoid hardcoding locale when linking to Microsoft

* Update additional note style in Windows guide

* More alignment for installation guide titles

* Apply suggestions from code review

* Fix typo and styling

Author: stevapple