Add OpenTelemetry webhook ingestion documentation

[maximelb]

Summary

• Adds a new tutorial page documenting how to use LimaCharlie webhook adapters as OTLP endpoints for ingesting OTel logs, traces, and metrics
• Covers setup (webhook creation, SDK/Collector configuration), event format reference for all three signal types, and D&R rule examples
• Added to mkdocs nav under Adapters > Tutorials, alongside the existing Webhook Adapter tutorial

Content

• Setup guide: Webhook creation, URL construction, OTel SDK env vars, OTel Collector YAML config
• Event format reference: Detailed field tables for logs, traces (spans), and all metric subtypes (gauge, sum, histogram, summary, exponential histogram)
• D&R rule example: Shows how to write detection rules against OTel events
• Backward compatibility note: Confirms existing webhooks are unaffected

Companion implementation PR: https://github.com/refractionPOINT/webhook-adapter-gateway/pull/27

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