ADS-B station docs and more

Having a wiki for the ADS-B station project is nice, but having all the docs in the project's repository is even better. Thus, I've moved the contents on the wiki into the dedicated docs directory inside the repository.

While at it, I tweaked the update-database.sh script. It now checks whether the OpenSky Network is up, and saves the previous version of the _the aircraft.db database under the aircraft.db.old name.

But wait, there is more! I also figured out how to convert the WMO weather codes into descriptions and add them to the daily forecast as well as the current weather.