fix: Explain necessary confirmation in GitHub side

According to discussion in the following issue, we found that it is necessary to enable three event
types. readthedocs/readthedocs.org#5135

Author: KengoTODA

src/rtd.ts

@@ -149,7 +149,8 @@ export default class RTD {
     }
     if (checkbox == null) {
       throw new Error(`:sob: Failed to visit version page at ${url}. ` +
-        "Please make sure you have enabled GitHub integration in your RTD project.");
+        "Please make sure you have enabled GitHub integration in your RTD project. " +
+        "Also confirm that [you selected three event types](https://docs.readthedocs.io/en/latest/webhooks.html#github) in GitHub config page.");
     }
     const checked = await (await checkbox.getProperty("checked")).jsonValue();
     return checked;