feat: add local technical docs.

[n1k0]

🔧 Problem

We need a place to write, maintain and serve technical documentation for the project. Gitbook is for methodology docs and doesn't support bi-directional git syncing, and Notion leaves a lot to be desired in terms of UX and browseability.

🍰 Solution

Use mdbook, a tool allowing to generate HTML docs out of Markdown files, the text file format we already know and use for other docs and which can be exported from Notion.

🏝️ How to test

Browse the generated HTML docs at https://ecobalyse-pr720.osc-fr1.scalingo.io/tech-docs

[n1k0]

I prefer closing this as this isn't high priority and exposes a lot more challenges hosting-wise than initially envisioned. And we don't have much technical stuff to write more that what already exists.