docs: give docs pages unique title tags and redirect stale pages (#4416)

## Summary

Several docs pages rendered identical `<title>` tags, which weakens
search indexing and makes results ambiguous. Each affected page now has
a unique, descriptive title while keeping its existing sidebar label
unchanged.

Alongside the retitles:

- Removed two stale build-system upgrade pages that were no longer in
the navigation, with redirects to the current package upgrade guide.
- Redirected the build-extensions group index to its overview page so
the two URLs stop sharing a title.
- Dropped a leftover orphaned API reference page (its old URL already
redirects to the management overview).

No links break: nothing in the docs points at the removed pages, and
every redirect target exists.

Author: D-K-P

docs/ai-chat/changelog.mdx

@@ -1,5 +1,5 @@
 ---
-title: "Changelog"
+title: "AI Agents SDK changelog"
 sidebarTitle: "Changelog"
 description: "Changelog for the AI Agents SDK."
 ---

docs/ai-chat/how-it-works.mdx

@@ -1,5 +1,5 @@
 ---
-title: "How it works"
+title: "How the AI Agents SDK works"
 sidebarTitle: "How it works"
 description: "End-to-end mechanics of a chat.agent turn: the two durable channels per session, the long-lived task that reads and writes them, and how a chat survives refreshes, deploys, and idle gaps."
 ---

docs/bulk-actions.mdx

@@ -1,5 +1,6 @@
 ---
-title: "Bulk actions"
+title: "Bulk actions in the dashboard"
+sidebarTitle: "Bulk actions"
 description: "Replay or cancel multiple runs from the dashboard using filters or selected run IDs."
 ---
 

docs/config/extensions/lightpanda.mdx

@@ -1,5 +1,5 @@
 ---
-title: "Lightpanda"
+title: "Lightpanda build extension"
 sidebarTitle: "lightpanda"
 description: "Use the lightpanda build extension to add Lightpanda browser to your project"
 tag: "v4"

docs/config/extensions/puppeteer.mdx

@@ -1,5 +1,5 @@
 ---
-title: "Puppeteer"
+title: "Puppeteer build extension"
 sidebarTitle: "puppeteer"
 description: "Use the puppeteer build extension to enable support for Puppeteer in your project"
 ---

docs/docs.json

@@ -721,6 +721,21 @@
     ]
   },
   "redirects": [
+    {
+      "source": "/config/extensions",
+      "destination": "/config/extensions/overview",
+      "permanent": true
+    },
+    {
+      "source": "/upgrading-beta",
+      "destination": "/upgrading-packages",
+      "permanent": true
+    },
+    {
+      "source": "/guides/new-build-system-preview",
+      "destination": "/upgrading-packages",
+      "permanent": true
+    },
     {
       "source": "/cli-dev",
       "destination": "/cli-dev-commands",

docs/guides/examples/lightpanda.mdx

@@ -1,5 +1,5 @@
 ---
-title: "Lightpanda"
+title: "Lightpanda examples with Trigger.dev"
 sidebarTitle: "Lightpanda"
 description: "These examples demonstrate how to use Lightpanda with Trigger.dev."
 tag: "v4"

docs/guides/examples/puppeteer.mdx

@@ -1,5 +1,5 @@
 ---
-title: "Puppeteer"
+title: "Puppeteer examples with Trigger.dev"
 sidebarTitle: "Puppeteer"
 description: "These examples demonstrate how to use Puppeteer with Trigger.dev."
 ---