Add FAQ for "Unknown Sources" flag to Firefox headset page (and Oculus Rift headset page)

It's mentioned in the Firefox instructions:

2. Ensure that your Oculus Home settings allow for Unknown Sources.

But we should draw more attention to this, as there have been some reports of the Unknown Sources flag getting unchecked.

Suggestions to make this more apparent to folks:

1. Add an FAQ item to remind users that Oculus > Settings > General > Unknown Sources should be enabled.
2. Embed the GIF (or just still screenshots) of the steps:

Updated gif: