fix(docs): keep the changelog and weekly archive reachable

Rames' review on #2978. Two pages left the sidebar without a redirect and
without being deleted, so they survived only as direct URLs: `docs/changelog.mdx`
and `docs/weekly-updates.mdx`.

Not deliberate, and the stack says so — #2979 upgrades `weekly-updates.mdx`,
importing DocsVideo and converting four raw <video> tags. You do not invest in
a page you meant to retire, and it carries `rss: true`, so it is a subscribable
feed. `product-updates.mdx`, which this stack adds to both the nav and the
footer, links to `/changelog` three times and `/weekly-updates` once. One of
those is advice to read the release archive before upgrading a production
workflow.

Both are back in the Explore group next to Product updates, which is where a
reader looking for "what changed" would go.

Worth naming why the verification missed it: the checker walks navigation → file,
which is why it correctly reported zero dangling entries. The file → navigation
direction — a page that exists, is not in the sidebar, and has no redirect — was
never checked, and that is exactly where these two sat.

`--check-redirects` on the existing `mint broken-links` step closes the adjacent
gap: it resolves every redirect destination, so a future restructure cannot
leave a redirect pointing at a page it removed. It does not catch the orphan
case above.

Also retargets `/guides/pipeline`. It pointed at `/concepts`, which explains how
a project is put together; the retired page was a seven-step process. `/workflows`
is the closer intent. The old step 3, "Strategy & Messaging", has no successor
anywhere in the docs — worth deciding deliberately rather than routing around.

.github/workflows/docs.yml

@@ -49,4 +49,6 @@ jobs:
 
       - name: Check broken links
         working-directory: docs
-        run: npx mint broken-links
+        # --check-redirects also resolves every redirect destination, so a
+        # restructure cannot leave a redirect pointing at a page it removed.
+        run: npx mint broken-links --check-redirects

docs/docs.json

@@ -64,7 +64,9 @@
             "group": "Explore",
             "pages": [
               "examples",
-              "product-updates"
+              "product-updates",
+              "weekly-updates",
+              "changelog"
             ]
           },
           {
@@ -750,7 +752,7 @@
     },
     {
       "source": "/guides/pipeline",
-      "destination": "/concepts",
+      "destination": "/workflows",
       "permanent": true
     },
     {