Document stations

[andythenorth]

#244 added station support in nml.

We need docs for stations.

As very few like writing docs alone, we're trialling collaborative doc editing, then converting to wiki later:

https://docs.google.com/document/d/1NHXIqofmgeXbqRZhFhJN52dNV-zMYaZEAUFevDvp1qs/edit

I also checked if GRF specification page needs updated; I don't think it does as nothing changed in OpenTTD / grf spec https://wiki.openttd.org/en/Development/NewGRF/Specification%20Status

[frosch123]

Looks like most variables were already documented in the wiki since 2012: https://newgrf-specs.tt-wiki.net/wiki/NML:Stations

I updated the page. Now it contains placeholders for all properties, variables, callbacks, and related constants.