Docs: add Logfire traceability and observability guide

[laura30llorente]

Summary

This PR introduces a complete guide for our observability stack using Pydantic Logfire.

It ensures current and future developers understand how to configure telemetry, properly instrument their code and navigate the Logfire dashboard for debugging and monitoring.

Changes included

• Configuration & Setup: Added instructions on how to set up logfire_config.py and configure the required .env variables, including a step-by-step visual guide to generating the LOGFIRE_TOKEN in the project settings.
• Code instrumentation: Documented the practical use of Logfire's core methods (span, info, warning, error, exception) with real code examples to illustrate best practices.
• Dashboard navigation: Added a visual guide with screenshots explaining how to locate and inspect nested spans and contextual variables within the Logfire Live/Explore views.
• Best practices: Included a clear checklist for maintaining end-to-end traceability when developing new features, agents or MCP tools.

Closes #31