Update HHVM FAQ

[AndrewDiMola]

https://docs.hhvm.com/hhvm/FAQ/faq

Some issues (please feel free to add):

• References old Wiki that is many years out of date
• Likely out of sync with actual products that use Hack and HHVM
• Needs a refresh -- open to ideas
• Should we remove reference to FastCGI? Not page, just reference.
• Assuming other issues that others can point out.

Maybe we should update the linked source as well:

• https://github.com/facebook/hhvm/wiki/Users

Longer term, we need to figure out what's worth keeping from the Wiki v. deleting. More of an issue when landing on the HHVM repo and reading the Wiki there.