document allowsBackForwardNavigationGestures and convertFilePath

The wkwebview-engile plugin had an `allowsBackForwardNavigationGestures` method that has been ported to cordova-ios when the plugin was integrated, but it's undocumented.

Also when the custom scheme was implemented, a `convertFilePath` method was added to convert file urls to scheme urls. But it's not documented neither.

They are both in `window.WkWebView` object.

Not sure if this should go in cordova-ios readme or somewhere else in cordova-docs.

Start with the window.WkWebView object and review the existing cordova-ios README and cordova-docs references to determine the appropriate documentation location. Document allowsBackForwardNavigationGestures and convertFilePath, then verify that both methods are discoverable in the chosen API documentation.

Written by the indexing model from the issue text.