Docs: add Traceability & Observability guide with Pydantic Logfire

[laura30llorente]

Summary

This PR introduces the official documentation for our observability stack using Pydantic Logfire.

It serves as the single source of truth for configuring, instrumenting, and scaling telemetry across all our Odoo addons.

Changes included

• Setup & Configuration: Added clear instructions for setting up logfire_config.py and the required .env variables.
• Code instrumentation guide: Provided explicit, code-backed examples of how and when to use logfire.span, info, warning, error, and exception.
• Scaling standards: Documented the step-by-step process for integrating Logfire into any future Odoo addons to ensure isolated service names.
• Dashboard navigation: Added a quick-start guide to help developers navigate the Live/Explore views and inspect nested spans.
• Best practices: Established strict guidelines for injecting Odoo context metrics into the logs.

Closes #7