Update docs

[balazsdukai]

• [x] include a proper first page
• [x] add a picture of the data in the first page
• [x] say that the data download (csv, gpkg, postgis) is for the whole country in one file
• [x] report the file size in the download section
• [x] structure the docs in a way so that's its clear for people who just want to quickly download the data and for people who are interested in the software
• [ ] So a LoD1 building can have varying ground heights for the different vertices?
• [x] explain that downloads are available for every month and the creation date is part of the file name

[balazsdukai]

Repo closes because it has been superseded.