Clean up or rewrite FAQ

[mattias-p]

• The FAQ should respect that a good part of its audience is non-technical.
• Answers should generally be quite short. Details not fitting in an FAQ should be pruned.
• The use of technical terms should be reviewed.
• Links should use reference link syntax instead of inline link syntax.
• Each sentence should start on new lines. (This helps maintenance e.g. by making diff outputs more readable.)
• Long sentences should be line-wrapped to limit line widths.
• Answers should be split into paragraphs where appropriate.
• A narrative text (e.g. a blog post) describing Zonemaster should be based on removed materials.

These items were derived from review comments in #101.

[pamasse]

@matsduf @sandoche2k Can you take a look on the no-technical part of the issue?
You can directly update the docs/FAQ/gui-faq-XX.md files

[sandoche2k]

@pamasse we will focus on this for 2020.1