Add FAQ topics to the Beats doc

[dedemorton]

This issue is for discussing topics that we might want to add to the Beats FAQs. Candidates for this list include questions that come up frequently in the discussion forums.

• How to get Filebeat to re-process log files.
• Why does Filebeat sometimes produce duplicates?
• How do I tell filebeat not to process older log files?

[dedemorton]

Closing because I think it’s better to focus on the core docs rather than making the FAQ longer (a short faq is better imo)