Doc features add

[julien-zgh]

Description:

This PR introduces human-readable feature specifications under doc/features/ to document the main TCR scenarios for onboarding, training, and real-life usage.

The goal is to make it easier to explain how TCR works, what users can expect from it, and how it is intended to be used, especially for new users and during coaching or training sessions.

The documentation is written in a Gherkin-like format to remain structured and readable, while staying documentation-first and non-executable for now.
This allows the project to learn from usage before deciding on a more technical solution (e.g. GoDog, literate tests, or living documentation).

No code behavior is changed and no new dependencies are introduced.

Fixes #810

Type of change

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• Bug fix
• New feature

Checklist:

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• My PR includes tests that cover my code changes.
  Documentation-only change; no executable tests added.
• Lint and formatter run with no errors
• All new and old tests are passing