Centralize documentation here

[TwinFan]

Current Situation / Problem Currently, to understand how XPMP2 works and what it offers, several places need to be read.

Suggested Solution Consolidate developer's documentation here as GitHub pages:

• [x] File format for xsb_aircraft.txt (this is currently scattered across kuroneko's wiki)
• [x] dataRefs supported by XPMP2 (this is partly at kuroneko's, partly at my fork)
• [x] Provide some general "how it works" background, can base on kuronekos wiki (but shorter) and on some of my additions
  • [x] Startup sequence
  • [x] Model Matching (see LT FAQ)
  • [x] TCAS support (again see LT documentation)
  • [x] AI/multiplayer support (LT has a bit)

Benefits Easier for developers to use (all features of) XPMP2.