feat: add headings styles in template doc

#399

Pour l'instant, j'ai fait ce qu'avait proposé Rémi #399 . On l'avait approuvé lors d'un daily

To check

• Explain what has changed in the changelog
• For newly created web components only : Declare all new angular proxy components in the file angular/projects/wcs-angular/src/lib/wcs-angular.module.ts
• For newly created web components only : Add the component name in Components-Overview.mdx with its corresponding image into assets/images/components-overview
• For newly created web components only : Add the component name in the left or right list in accessibility.mdx

Design token migration (specific task)

• check that css variables for which you change the name are not setter / read in typescript code. Rename them in the ts code if necessary.
• Document all renamed css variables in the migration guide.
• Document all component token (css variable) in the component doc (with ts annotation) so that they can be used by the projects.