Create a quarto reveal JS insee-clair extension

- In a dedicated branch, turn the existing repo into a working quarto extension.

- Read quarto documentation to create an extension from scratch : https://quarto.org/docs/extensions/creating.html
- in dedicated branch, run quarto create extension
- create an _extension.yml file with following fields :
```
title: insee-clair
author: "Insee (d'après Romain Puyau)"
version: 0.1.0
quarto-required: ">=1.4.0"
contributes:
formats:
revealjs:
theme: [default, ressources/Insee_Commun.scss]
css: [ressources/css/insee_clair.css]
title-slide-attributes:
data-background-image: _extensions/insee/ressources/1_Template-INSEE-Clair_premiere-page.svg
data-background-size: contain
data-background-opacity: "1"
toc: false
template-partials:
- toc-slide.html
- toc-style.html
filters:
- toc-filter.lua
slide-number: true
footer: "Insee"
```
- move ressources necessary to the extension folder :
- img to _extensions/insee-clair/resources/img
- css to _extensions/insee-clair/ressources/css
- modulesCompl to _extensions/insee-clair/ressources/modulesCompl
- update relative paths in _extension.yml
- import the template, update the format output, check it's working with quarto preview template.qmd

Other :
- add a .quartoignore :
```
index.qmd
_quarto.yml
_variables.yml
dev.R
_output/
slides/
CHANGELOG*
*.html
```
- update README with a one-line description, the install commands, the format name, and a customization pointer. Skeleton:
```
# quarto-insee-extension

Template Quarto reveal.js intégrant la charte graphique Insee 2025 (thème clair).

## Installation (nouveau projet)
\```bash
quarto use template InseeFrLab/quarto-insee-extension
\```
## Installation (projet existant — extension seule)
\```bash
quarto add InseeFrLab/quarto-insee-extension
\```
## Utilisation
Dans l'en-tête YAML : `format: insee-clair-revealjs`.
Personnalisation : `_extensions/insee-clair/ressources/css/` et `ressources/Insee_Commun.scss`.
```
- licence : etalab 2.0

- The extension should have a template named template.qmd. See further explanations in https://quarto.org/docs/extensions/starter-templates.html
- Test the install path with quarto use template*