doc: Possible entries and options for a settings menu

Since OpenAge will support more options than the original game, we likely have to redesign the entire settings menu from scratch. Before we do that, it would be beneficial to enumerate all entries for our menu. Layouters or designers can then use this information to draft a GUI, e.g. for #728 .

The options we have to enumerate (as suggested by @coffenbacher) are:

• Game Settings
  • all useful settings available up through the latest UP / Voobly / HD
  • any settings we see demand for that aren't even in UP but will highly likely be in OpenAge
  • any settings for any highly likely OpenAge-specific features
• Multiplayer Game Options
  • all useful settings available up through the latest UP / Voobly / HD
  • any settings we see demand for that aren't even in UP but will highly likely be in OpenAge
  • any settings for any highly likely OpenAge-specific features

Additionally it would be nice, if we could sort all entries into categories.

Inspiration can also be drawn from other (RTS-)games with nice and highly configurable interfaces like DoTA2.

Feel free to suggest any options/entries/categories/nice-to-have-features in the comments. I'll add everything to this gist.

Review the existing checklist in this issue and the linked gist; compare the requested Game Settings and Multiplayer Game Options with relevant RTS interfaces, including the examples named here. Done means the entries are enumerated, organized into categories, and added to the gist for designers working on #728.