docs: restore #custom-files anchor on llms-txt overview page

fern/products/docs/pages/ai/llms-txt/overview.mdx

@@ -27,6 +27,18 @@ Both files are available at any level of your documentation hierarchy (`/llms.tx
 
 Examples: [Eleven Labs llms.txt](https://elevenlabs.io/docs/llms.txt), [Cash App llms-full.txt](https://developers.cash.app/llms-full.txt).
 
+## Custom files
+
+To serve your own `llms.txt` or `llms-full.txt` instead of the auto-generated versions, point to your files under the [`agents` key in `docs.yml`](/learn/docs/configuration/site-level-settings#agents-configuration):
+
+```yaml docs.yml
+agents:
+  llms-txt: ./path/to/llms.txt
+  llms-full-txt: ./path/to/llms-full.txt
+```
+
+Custom files are served at the root-level `/llms.txt` and `/llms-full.txt` endpoints. Any file you don't specify falls back to the auto-generated version. See [Serve custom files](/learn/docs/ai-features/customize-llm-output#serve-custom-files) for the full reference.
+
 ## Page descriptions
 
 Both files include page descriptions pulled from [frontmatter](/learn/docs/configuration/page-level-settings). Fern uses the `description` field if present, otherwise falls back to `subtitle`.