Create toy city for testing and docs

The toy city should cover all edge cases, including:
- separate street component
- separate bike component
- with separate cycleway
- with onstreet bike infra
- with loops and parallel lines on bike network
- with loops and parallel lines on street network
- with parallel bike and street lines, bike slightly longer
- with parallel bike and street lines, street slightly longer
- with bike infra longer and shorter than 100m

to:
- [ ] test all network simplifications/filters/merges and seed point filters/snaps
- [ ] showcase on the data import for City definition docs page that an entire non-osm custom city can be loaded, new subsection
- [ ] provide a guided step by step insight to developers on a new developer docs page showing which variables mean what and how they relate and are constructed from each other. Best with a diagram like here https://osmcode.org/osmium-tool/manual.html#creating-geographic-extracts

Because it's getting pretty complicated.