Convert `doc/` to markdown
audio: removed leftover mergeconflict files
gitignore: ignore mergeconflict .orig files
testing: fixed memory errors at sound and texture filenames
doc: code style updates

updated pretty much everything and added more comments.
Merge branch 'markdown-doc' of https://github.com/goto-bus-stop/openage
doc: also export private members in generated docs
convert: fixed output path for the generic file export

slp and wav files were not affected, the fallback handler
wrote to the root directory instead of the requested data folder
convert: export all files from the data files
convert: new intel on graphics speed adjustment and civ graphic set
convert: format strings for filename debug output
engine: optimized comment formatting
doc: optimized formatting and whitespace
doc/media: blending mode it tables updated and clarified
doc/media: gamedata storage documentation started
doc: adapted milestones and tech_spec to the current project status
engine: hotspot alignment for all buildings pretty much fixed

still buggy in some cases, not sure if that's the build data's fail.