SSG/Prerendering: Allow generating foo.html instead of foo/index.html

### Description

Angular's Prerendering puts nested routes into a subfolder with `index.html`:
*=> Route `blog/myslug` becomes `blog/myslug/index.html`*

**The web server adds a trailing slash (for the folder) which is then removed by the Angular router, and it leads to an initial redirect.**

### Describe the solution you'd like

Some static site generators allow to create a flat structure instead with no subfolder:
*=> Route `blog/myslug` becomes `blog/myslug.html`*

**Proposed solution:**
- Output `myslug/index.html` by default, it is the most compatible output that works on each and every web server
- add an option for flat output (`myslug.html`) that can be used if I know my web server supports this

### Context

- Jekyll always outputs `myslug.html`
- [Nuxt v2 has an option](https://nuxt.com/deploy/cloudflare#route-matching)
- [Next.js has an option](https://nextjs.org/docs/app/api-reference/config/next-config-js/trailingSlash)

Most cloud hosting providers allow to serve `myslug.html` under `myslug`:
- GitHub Pages
- Cloudflare Pages
- [Firebase Hosting](https://firebase.google.com/docs/hosting/full-config#control_html_extensions)
- [Vercel](https://vercel.com/docs/projects/project-configuration#cleanurls)
- [Netlify](https://docs.netlify.com/routing/redirects/redirect-options/#trailing-slash)

Further read on trailing slash and framework/provider support: https://bjornlu.com/blog/trailing-slash-for-frameworks

Commencez par le point d’entrée du prerendering de Angular CLI et examinez les tests existants pour la sortie des routes imbriquées. Confirmez la structure actuelle de index.html, puis définissez et vérifiez une option qui produit une structure plate de route.html tout en préservant le comportement par défaut.

Rédigé par le modèle d'indexation à partir du texte de l'issue.