Split documentation into separate files

docs/developement.md

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+## Developement
+
+```sh
+# Using yarn (recommended)
+yarn install
+yarn serve
+
+# **OR** Using npm
+npm install
+npm run serve
+```
+
+### Themes
+
+Themes are meant to be simple customization (written in [scss](https://sass-lang.com/documentation/syntax)).
+To addd a new theme, just add a file in the theme directory, and put all style in the `body #app.theme-<name>` scope. Then import it in the main style file.
+
+```scss
+// `src/assets/themes/my-awesome-theme.scss`
+body #app.theme-my-awesome-theme. { ... }
+```
+
+```scss
+// `src/assets/app.scss`
+// Themes import
+@import "./themes/sui.scss";
+...
+@import "./themes/my-awesome-theme.scss";
+```