docs: Update FAQ
docs: Update FAQ

- Add "What is Firecracker?" as the first question
- Give a better explanation and history about why we built Firecracker
- Change the "is Firecracker compatible" question and answer a bit
- Other misc. changes

Signed-off-by: Matt Wilson <260f5074942ca36dab941199ff6b19fcaa26ccde@amazon.com>

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