Improve documentation

[marctc]

We identified some areas where our documentation needs to be extended:

• [Docs] Security (root access, privileged mode, Linux capabilities) #1404
• [Docs] Performance overhead (CPU and memory) #1405
• [Docs] Metrics cost: Expected cardinality #1406
• [Docs] Tracing: What works and what doesn't #1407

[grafsean]

@marctc I think each can be a new top-level article in docs/sources. I left a comment on each issue with a suggested file and topics.

Next steps, could you get your draft content in to the files. I'm available this week to review and on PTO next week. I can also complete a format and copy edit when I return in early February.