Make installation pages more complete

[Gymnasiast]

Currently, most of the documentation is spread out around openrct2.org, openrct2.io, the GitHub wiki, Reddit posts and docs.openrct2.io. The goal is to concentrate all this documentation in one place (and link to it from the older sources). Also, much of the documentation is written in a quite generic way, while the docs can give more specific instructions for every operating system, hopefully increasing its user-friendliness.

• Windows
  • Installing OpenRCT2
  • Retrieving files from RCT2
  • Retrieving files from RCT Classic + Mini Game
  • Retrieving files from RCT1
• macOS
  • Installing OpenRCT2
  • Retrieving files from RCT2: disc
  • Retrieving files from RCT2: GOG
  • Retrieving files from RCT2: Steam
  • Retrieving files from RCT Classic + Mini Game
  • Retrieving files from RCT1: disc
  • Retrieving files from RCT1: GOG
  • Retrieving files from RCT1: Steam
• Linux
  • Installing OpenRCT2
    • Distro-specific
    • AppImage
    • Compiling yourself
  • Retrieving files from RCT2: disc
  • Retrieving files from RCT2: GOG
  • Retrieving files from RCT2: Steam
  • Retrieving files from RCT Classic + Mini Game
  • Retrieving files from RCT1: disc
  • Retrieving files from RCT1: GOG
  • Retrieving files from RCT1: Steam

[da5nsy]

Much of this has been done now 🎉 🙌 (If I get a chance I might work through the list and check what remains, at some point...)

Flagging one bit that seems to have slipped through:
The Windows install instructions are lacking a link to: https://openrct2.io/
(something like this section in the macOS section)