docs(trailingSlashes): add note for SSG generation (#57628)

When trying to generate my SSG docs site I couldn't figure out why the
navigation links (`href="/about`) were not mapping to the generated
files (e.g. `/out/about.html)`.

`trailingSlash` was key to converting the files to
`/out/about/index.html` to make the SSG links work properly.

I've updated the trailingSlash docs to be clear how they affect SSG
mode.

It might also make sense to add this to the SSG guide as well.

Co-authored-by: Steven <steven@ceriously.com>

docs/02-app/02-api-reference/05-next-config-js/trailingSlash.mdx

@@ -17,6 +17,8 @@ module.exports = {
 
 With this option set, urls like `/about` will redirect to `/about/`.
 
+When used with [`output: "export"`](/docs/app/building-your-application/deploying/static-exports) configuration, the `/about` page will output `/about/index.html` (instead of the default `/about.html`).
+
 ## Version History
 
 | Version  | Changes                |