Handle informal sidepath

This is just to document the case. I don't have an suggestion on how to handle this; nor do I think it's important to handle now.

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Sometimes there are informal sidewalks next to a road.
If those are mapped, marking them as `informal=yes` (DE: Trampelpfad) is a feasable tagging.

I am not clear on what the right `sidewalk`-value be in this case. It seems similar to the situation of https://trafficsigns.osm-verkehrswende.org/DE?signs=DE:239,1022-10 where bikes are allowed to use the (pedestrian) sidewalk (at walking speed) but this is not considered bike infra so it still gets a cycleway:SIDE=no on the centerline.

For the sake of routing, it would be better to recognize those `informal=yes` "`is_sidepath`" as some kind of infra and not generate a virtual sidewalk.

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Examples

* https://www.openstreetmap.org/way/1007134506/history
(Note I just changed the tagging to improve it a bit: better highway class `footway` but especially `is_sidepath`)

* There are more cases in this general area where the "sidewalk" is a dirt path
https://www.openstreetmap.org/way/4935223 In this case someone marked them as `sidewalk=both`

Das Issue beschreibt eine Tagging-Mehrdeutigkeit für informelle Nebenwege in OSM. Um die Codebasis zu verstehen, suche nach der Logik zur Generierung von Gehwegen und danach, wie die Tags `is_sidepath` und `informal=yes` derzeit behandelt werden. Prüfe den Code des Repositorys auf Gehwegrouting und die Interpretation von Tags. Ziel ist es, eine Lösung vorzuschlagen, nicht eine zu implementieren; recherchiere daher die bestehende Behandlung ähnlicher Tags und erwäge, wie die Generierung virtueller Gehwege angepasst werden könnte.

Vom Indexierungsmodell aus dem Issue-Text verfasst.