Expand installation, deployment, and troubleshooting documentation

[httpdss]

Summary

Create or improve installation and deployment docs for the most common collectd-web setups.

Proposed docs

• Debian/Ubuntu package install.
• Standalone service usage.
• Nginx reverse proxy setup.
• Apache setup.
• Basic collectd/RRD directory expectations.
• Troubleshooting common graph/path/dependency problems.

Why this matters

Install friction blocks adoption. Clear deployment docs also create SEO surfaces for users searching specific setup questions.

Acceptance criteria

• Docs include copy-paste commands where possible.
• Each setup explains assumptions and expected paths.
• Troubleshooting section covers missing graphs, permissions, missing RRD data, and service startup issues.
• README links to the install/deployment docs.

[github-actions]

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