✨(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/404.spec.ts

@@ -10,8 +10,6 @@ test.beforeEach(async ({ page }) => {
 
 test.describe('404', () => {
   test('Checks all the elements are visible', async ({ page }) => {
-    await expect(page.getByLabel('Image 404')).toBeVisible();
-    await expect(page.getByText('Ouch')).toBeVisible();
     await expect(
       page.getByText(
         'It seems that the page you are looking for does not exist or cannot be displayed correctly.',