docs page for 'how to open picture in picture video in firefox'

[jwunderl]

Describe the bug

Firefox doesn't use the standard 'picture in picture' api, so we have to guide users on how to do that themselves. We need a arcade.makecode.com/firefox-picture-in-picture page with instructions on how to do that (like a targeted version of the section from https://support.mozilla.org/en-US/kb/about-picture-picture-firefox), which the button from microsoft/pxt#9298 will point at~