Rearrange documentation

1) Moves all READMEs to docs/
2) Renames them to *.md, adds some Markdown with the idea to add a lot more
3) Moves the doxyfile config to doc/ and makes it parse the headers at ../include as well as the md files in docs.
4) Skips SDL_opengl*.h headers from the docs
5) Minor fixes to doxyfile