docs: expand getting-started plugins chapter

[Taure]

Summary

• Document all nova_request_plugin options (decode_json_body, read_urlencoded_body, parse_qs) with types, request map keys, and usage examples
• Add new sections for nova_correlation_plugin and nova_csrf_plugin with full option tables and examples
• Introduce per-route plugin overrides with router examples
• Fix incorrect "lowest priority number first" ordering claim — plugins run in list definition order
• Replace vague "see Nova documentation" link with a concrete summary table of all 4 built-in plugins
• Add realistic multi-plugin sys.config example with ordering rationale

Test plan

• mdbook build succeeds with no errors
• Plugin options cross-referenced against Nova source (nova_request_plugin.erl, nova_correlation_plugin.erl, nova_csrf_plugin.erl)
• Review rendered output for readability

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