Add missing-redirects check mention to redirects and slug pages

 (#5177)

Co-authored-by: fern-api[bot] <115122769+fern-api[bot]@users.noreply.github.com>
Co-authored-by: Devin Logan <devinannlogan@gmail.com>

Author: fern-api[bot]

fern/products/docs/pages/changelog/2026-04-02.mdx

@@ -15,18 +15,20 @@ agents:
 
 ## Missing redirects check rule
 
-The `missing-redirects` rule for `fern check` detects pages that were removed or moved without a redirect. It compares your current docs navigation against the previously published state and warns when a previously published URL would return a 404. Configure its severity in `docs.yml`:
+The `missing-redirects` rule for `fern check` detects pages that were removed or moved without a redirect. It compares your current docs navigation against the previously published state and warns when a previously published URL would return a 404.
+
+```bash title="Example" wordWrap
+[warning] Page "products/api-def/openapi/extensions/default.mdx" was removed. The previously published URL "/learn/api-definitions/openapi/extensions/default-values" will return 404 without a redirect. Consider adding a redirect in docs.yml to preserve existing links.
+```
+
+Configure its severity in `docs.yml`:
 
 ```yaml docs.yml
 check:
   rules:
     missing-redirects: error
 ```
 
-Requires authentication via `fern login` or the `FERN_TOKEN` environment variable. The check is skipped on first publish, when unauthenticated, or when the network is unavailable.
-
-Requires Fern CLI version `4.57.0` or later.
-
 <Button intent="none" outlined rightIcon="arrow-right" href="/learn/docs/configuration/site-level-settings#check-configuration">Read the docs</Button>
 
 ## Library docs generator

fern/products/docs/pages/seo/configuring-slugs.mdx

@@ -47,6 +47,10 @@ In the example above, the **Welcome** page would be hosted at `plantstore.docs.b
 
 You can customize these default slugs by renaming them or skipping them entirely.
 
+<Note>
+  Changing a slug updates the page's URL. Run [`fern check`](/learn/cli-api-reference/cli-reference/commands#fern-check) to detect pages that moved without a [redirect](/learn/docs/seo/redirects#catching-missing-redirects), so existing links don't break.
+</Note>
+
 ## Renaming slugs
 
 Set the `slug` property in `docs.yml` or in a page's frontmatter to customize the URL path.

fern/products/docs/pages/seo/redirects.mdx

@@ -12,12 +12,16 @@ subtitle: Learn how to configure redirects in Fern Docs. Set up exact path redir
   To add external links to your sidebar navigation, see [Navigation](/learn/docs/configuration/navigation#links).
 </Tip>
 
+## Catching missing redirects
+
+You can use [`fern check`](/learn/cli-api-reference/cli-reference/commands#fern-check) to automatically detect pages that were removed or moved without a redirect. Configure the [`missing-redirects` rule](/learn/docs/configuration/site-level-settings#check-configuration) in `docs.yml` to control its severity.
+
 <llms-only>
 ## Common errors
 
 Errors below are surfaced by `fern check` and `fern generate --docs`.
 
-### Page "X" was moved from "/old" to "/new". The old URL will return 404 without a redirect.
+### Page "X" was moved from "/old" to "/new". The old URL will return 404 without a redirect. Consider adding a redirect in docs.yml to preserve existing links.
 
 A page's [slug](/learn/docs/seo/configuring-slugs) changed relative to the last [published version](/learn/docs/preview-publish/publishing-your-docs) of your docs. Add a [redirect](/learn/docs/configuration/site-level-settings#redirects-configuration) in `docs.yml` so existing links keep working:
 
@@ -27,7 +31,7 @@ redirects:
     destination: /new
 ```
 
-### Page "X" was removed. The previously published URL "/old" will return 404 without a redirect.
+### Page "X" was removed. The previously published URL "/old" will return 404 without a redirect. Consider adding a redirect in docs.yml to preserve existing links.
 
 A [published page](/learn/docs/preview-publish/publishing-your-docs) no longer exists in the [navigation](/learn/docs/configuration/navigation). Add a [redirect](/learn/docs/configuration/site-level-settings#redirects-configuration) to another relevant page to avoid breaking incoming links: