Document remote API key, UnifiOS terminology, and scrape interval limits

[platinummonkey]

Summary

• Documents the Remote (Site Manager) API key login method, including a configuration example and a Prometheus multi-site scrape example (Documentation updates #55)
• Documents the local controller API key option as an alternative to user/pass (Documentation updates #55)
• Replaces the UDM/UXG/UDM-Pro/UCK device list with UnifiOS-based language, since UnifiOS now runs on more devices and can be self-hosted (Documentation updates #55)
• Clarifies that UnifiOS controllers work on port 443 (no :8443 needed) vs. non-UnifiOS controllers, which still need :8443 (Documentation recommendation #42)
• Adds a note distinguishing the Prometheus namespace setting from the controller user setting, since these are unrelated and easy to conflate (Documentation recommendation #42)
• Adds an FAQ entry explaining that the UniFi controller API doesn't update every value on every request, so lowering the Prometheus scrape interval doesn't increase resolution, and recommends a rate() range of at least 3m to avoid spurious zeros (closes the outstanding question on Grafana Dashboards have 'dips' #24)

Test plan

• yarn build in documentation/ completes with no broken-link warnings
• Spot-check rendered pages (gettingstarted, controllerlogin, configuration, prometheus, faq) via yarn start