Tagging error when sidewalk:*=separate, but not sidewalk present

In the list of tagging errors in the `README.md` an important tagging error is missing: A street is tagged with `sidewalk:both=separate` (or something similar), but there is no separate sidewalk mapped. This typically causes routers to make detours, because the can't find a way along this street.

Some [examples](https://www.openstreetmap.org/#map=17/50.813077/7.176105) can be found in Troisdorf. (I added notes there.)

The issue is about missing tagging errors in README.md. Start by examining the code that generates tagging errors, likely in a validation module. Look for existing error checks and add a new one for sidewalk:both=separate without mapped sidewalks. Test with the provided OSM examples in Troisdorf to verify the error detection works.

Written by the indexing model from the issue text.