✨(frontend) create generic theme

By default Docs will not be on the dsfr theme but
on the generic theme. La Gaufre is part of the dsfr
theme and is removed from the generic theme.
Same for the "beta" keyword and the "proconnect"
buttons.

src/frontend/apps/e2e/__tests__/app-impress/doc-grid.spec.ts

@@ -208,7 +208,6 @@ test.describe('Documents filters', () => {
 
     // Initial state
     await expect(allDocs).toBeVisible();
-    await expect(allDocs).toHaveCSS('background-color', 'rgb(238, 238, 238)');
     await expect(allDocs).toHaveAttribute('aria-selected', 'true');
 
     await expect(myDocs).toBeVisible();