feat: Option to export as GeoJSON

**Is your feature request related to a problem?**
It has been a recurring issue at the bikenetkit.org development that for sb we only have gpkg exports - for technical reasons, see: https://github.com/BikeNetKit/dataexport/issues/37. Also, geojson is a widely understood format, while gpkg is mostly just understood in the geospatial community, so this might also help the future uptake of sb. For such reasons, in all other bikenetkit packages we have the option for both geojson and gpkg exports.

**Proposed Solution**
Implement an option for geojson export, see https://github.com/BikeNetKit/dataexport/issues/37

**Alternative Solutions**
Over at https://github.com/BikeNetKit/dataexport we could write a conversion script to turn the existing gpkg output into geojson. But why not support it natively, given it makes a lot of sense.

**Use Cases**
See above

**Additional Context**
We are in the data export phase for bikenetkit.org, so we would need this feature soon, like within 2 weeks. BUT I don't want to create any stress to you @cbueth! You might be the best to develop this feature, but I totally understand if you don't have the time for it. Could you please let me know, so I can plan whether we should write a conversion script. This wouldn't be a problem, just thinking it's better to avoid doing the same work twice, if it's possible.

**Willing to Contribute?**
- [ ] I'm willing to submit a PR for this feature
- [X] I can help test this feature
- [X] I can help document this feature

Start by tracing how the existing GPKG export is produced and review the linked dataexport issue for the expected GeoJSON behavior. Add a native option for GeoJSON export while preserving the current GPKG output, then confirm the resulting files work for the stated export use cases and document the option.

Written by the indexing model from the issue text.