[WIP] Write the full documentation of the framework

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Write a comprehensive, well-written, state of the art documentation for the "PitouFW", a PHP framework for those who didn't want a framework in the first place.

The documentation you will provide must consist in a README.md file at the root of the project, and a docs/ folder containing the complete reference for all the classes, helpers and APIs of the framework.

Good luck and have fun!

Fixes #4.

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