add --docs flag for utility
ship a proper SDK for modders

including the OpenRA.TilesetBuilder.exe fixes #2316
and auto-generated trait documentation fixes #2437
compile Markdown to HTML for offline use
purge external files from the doc folder (moved to Wiki)
remove unused non-free Dune 2000 font
embed the previously manually edited outdated documentation

from https://github.com/OpenRA/OpenRA/wiki/Trait-Documentation
automatically update the wiki documentation each release
Merge pull request #2808 from Mailaender/trait-documentation

Auto-Generated Trait Documentation