Explain better the risks of documents and how they can be malicious

[deeplow]

Some inspiration about malicious PDFs here: https://github.com/jonaslejon/malicious-pdf

Notes / Evidence

During user testing, we learned that it was not obvious for participants some what risks working with documents presented.

[apyrgio]

Malware and metadata removal are two important features of Dangerzone that should be the very first thing that a user sees. It would make sense to briefly summarize them in the front page (see #19), and add a link to an FAQ section with a more detailed explanation.

(Similar approach to #18)