Add comparison docs: collectd-web vs Grafana / heavier observability stacks

[httpdss]

Summary

Publish an honest comparison page explaining when collectd-web is a good fit versus a larger observability stack such as Grafana.

Why this matters

Many users searching for monitoring dashboards default to Grafana. collectd-web can win when the user wants a simple, lightweight frontend for existing collectd/RRD data.

Scope

• Add a docs page such as docs/collectd-web-vs-grafana.md.
• Compare setup complexity, resource footprint, data source assumptions, customization, and best-fit use cases.
• Include “choose collectd-web when…” and “choose Grafana when…” sections.
• Link from README/docs navigation.

Acceptance criteria

• Comparison is balanced and technically accurate.
• Page positions collectd-web around lightweight collectd/RRD visualization rather than claiming to replace full observability platforms.
• Page includes CTA to install/test collectd-web.
• README links to the comparison page.