add FAQ entry for 404 link errors

src/pages/faq/index.md

@@ -5,6 +5,16 @@ description: A guide with FAQ about the ADP Developer Site
 
 # ADP Developer Site Best Practices
 
+## Why is my link returning a 404?
+
+A few common causes:
+
+- **Trailing slash** — pages served from `index.md` files require a trailing slash. The no-slash version won't redirect unless a redirect is explicitly configured. See [Paths and Links](https://developer.adobe.com/dev-docs-reference/getting-started/dev-docs/best-practices/#paths-and-links).
+- **Path characters** — paths use hyphens, not underscores or periods. See [File and Directory Naming](https://developer.adobe.com/dev-docs-reference/getting-started/dev-docs/best-practices/#file-and-directory-naming).
+- **Expired redirects** — after migrating from Gatsby, we add temporary redirects to cover old bookmarks. Those are removed after ~4–6 weeks. New pages never get them, so there's no fallback. See [Paths and Links](https://developer.adobe.com/dev-docs-reference/getting-started/dev-docs/best-practices/#paths-and-links).
+
+To detect 404s in your repo, run the link checker commands in your [`package.json`](https://github.com/AdobeDocs/dev-docs-template/blob/d67b86f4dca4f30ecb9825fe7dd08b392475ad4b/package.json#L27-L28).
+
 ## How do I link PDF or ZIP files for download or viewing?
 To host and link PDF files (or other files like `ZIP` or `.d.ts`), use a URL and use relative path to file within `src/pages`: