Add a readme for the web/ subdirectory

web/README.md

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+<p align="center">
+    🖥&nbsp;&nbsp;<strong><em>The Web Application</em></strong>&nbsp;&nbsp;🖥️
+    <br>
+    <br>
+    <strong>Interested in Contributing?</strong> Take a look at <a href="https://docs.onyxia.sh/contributing/onyxia">our technical documentation</a>
+    <br>
+    <br>
+</p>
+
+## Overview
+
+This repository contains the source code for the Docker image [inseefrlab/onyxia-web](https://hub.docker.com/r/inseefrlab/onyxia-web).
+
+## Architecture
+
+`onyxia-web` is a Single Page Application (SPA) that runs entirely in the user's browser and is delivered as static files. 
+
+## Project Structure
+
+- **UI Layer:** This project utilizes React, but solely as a UI library. The React-specific code is isolated to [src/ui](./src/ui).
+
+- **Core Logic:** The bulk of the application's functionality resides in [src/core](./src/core). Importantly, the core logic is entirely agnostic to React.