feat: create general privacy page and platform specific ones

[ermshiperete]

Currently the only privacy policy page we have is for Android and we point users to that page for all platforms.

We should extract the platform independent bits to a general privacy policy page, and also add a privacy policy page to each platform that explains how to disable crash reporting on that platform etc.

From discussion on Slack.

[darcywong00]

Adding a note that the source to the Android privacy page is maintained in the main repo:
https://github.com/keymanapp/keyman/blob/master/android/docs/help/about/privacy.md