Add a "Known DNS Providers" page

[ameshkov]

Proposed layout:
https://gist.github.com/ameshkov/b904a5bf7bb162bbc4d95c6f1ea1b89e

This page should be added under "General questions" (as it will be reused in other AdGuard products).
@admitrevskiy the recommended DNS providers from our Android app should be also listed there.

This needs to be done asap:
AdguardTeam/AdguardForAndroid#2145 (comment)

Here is a list of the providers we should add there in the first place:
AdguardTeam/AdguardForAndroid#2374 (comment)