Update FAQ

[VoidingWarranties]

• mining pool answer out of date
• "How do I upload a file" answer out of date
• "How do I download a file?" answer out of date
• "Can I remove a file after uploading to Sia?" answer is no longer correct
• "Can I see how many times a file has been downloaded?" contains a reference to Sunfish, which no longer exists.
• "Can I share files with others on Sia?" is out of date, but maybe it's not worth updating since we plan to add that feature back in anyways.

Others are probably out of date as well.

Remember to mark each outdated translation with a # TODO: update translation and file an issue for the corresponding language with the "needs translation" label.