Documenting the Mobile API Response JSON and Untis HTML

[pixelcmtd]

The server then responds with a JSON array of entries that are reasonably complex, but if you just want the URLs to the substitutions: [...]

Thanks in part to Ampless/dsbuntis#10, I think that I now understand, how the Mobile API's JSON looks. While adding support for more parts of it to dsbuntis, this project should also include some more documentation on it.

Parsing the plans from the HTML depends on the format of them, which usually is like Untis always does HTML. But the HTML format is not documented here, because it does not depend on the DSB API used.

That should also change, as we get a better idea of how it looks.