[FAQ] Merge wiki FAQ into main FAQ

Merge content of https://github.com/crosswalk-project/crosswalk-website/wiki/FAQ
into main FAQ on website.

Fixes https://crosswalk-project.org/jira/browse/XWALK-1686
[FAQ] Set headings at correct level
[FAQ] Group sections more intuitively
[FAQ] Remove slightly esoteric section about CEF/QtWebKit
Merge pull request #139 from crosswalk-project/faq-merge-1686

[FAQ] Merge wiki FAQ into main FAQ