Support for static assets copying in web generator for SSG support

### **Description:** As doc-kit moves towards SSG, we need to handle static assets like logos, so when any project use doc-kit, like webpack, can have its images. now, doc-kit don't handle that so the images crash. ### **Solution:** Add `staticDir` option in configration file, so doc-kit can enter that file and copy recursively every folders and static assets. Copying phase will be in file [generate.mjs](https://github.com/nodejs/doc-kit/blob/main/src/generators/web/generate.mjs) after CSS bundle line.