Move code out of MD files

[mkiedys]

This is follow-up from PR #125:

@viktorklang wrote:
(...) what I meant was that I think that sample code should live in compiled files, so that we can catch when it needs to be updated. As such I think all code should be moved out of .mds and put into files and then either imported automatically into the .mds or be linked from the .mds.

[ktoso]

I was looking into this today and sadly github is still working on securing markup includes github/markup#172 (comment) and it is currently not possible to do without pre-processing.

[rkuhn]

Checking: no change on the github side of things. This is a generic ticket with possibly a different solution, so if somebody has a low-overhead working principle to point at then we should definitely try it out.

[viktorklang]

I think we should just link to the example files or the spec classes (now with Java they have their own files).

[ktoso]

@reactive-streams @viktorklang so are we planning on doing something with this now?
Tech wise the situation has not changed, we can't include files.

[viktorklang]

I think this will stay as a "nice-to-have".

[ktoso]

Sounds good to me.

[viktorklang]

I don't think there's enough need to warrant this Issue. If someone disagrees, please reopen.