I get dozens existing normal pages, not a single bad result: https://duckduckgo.com/?q=arduino+doorbell+optocoupler
You should check your computer for viruses, maybe upgrade the operating system and browser, select a better browser, and perhaps a better DNS service with a filter, and select a more normal search engine.
If you don't know if it is AC or DC and don't know the polarity, then you need a full-bridge-rectifier plus a resistor, or else just a single diode plus a resistor before the opto-coupler.