To add yandex map services ( present in mercator projection by default) about xyzservices

[martinfleis]

@krishnaduttPanchagnula Do you know where are Yandex maps documented? We need to ensure that their license allows at least some limited use in non-commercial open source.

If you know that they do (and can ideally link to that), you can try to add it yourself as addition to provider_sources/xyzservices-providers.json following our contributing guide:
https://xyzservices.readthedocs.io/en/latest/contributing.html#providers

[krishnaduttPanchagnula]

Here is the documentation for usage policy and api integration : https://yandex.com/dev/maps/mapsapi/

These are terms of Use:
1.All map data must be used on a website or application that is freely available to access. You are not allowed to store or change the data, but you may cache geocoding and routing requests for up to 30 days.
2.The free API is not for use in closed systems or for tracking transport.
3. The maximum number of total requests to the geocoder, router, and panoramas is 25,000 per day.

[martinfleis]

That sounds great. Would you make a PR adding it?