add redirect for old upgrading.html URL to fix broken changelog links (#20582)

## Which issue does this PR close?

- Closes #20572

## Rationale for this change

When upgrade guides were split into separate pages (#20183), the old
`upgrading.html` URL broke. All changelog files still reference this old
URL, causing 404 errors for users.

## What changes are included in this PR?

Added a redirect in `docs/source/conf.py` using the existing
`sphinx_reredirects` extension to redirect
`library-user-guide/upgrading.html` to
`library-user-guide/upgrading/index.html`.

This preserves all existing changelog links without needing to update
historical files.

## Are these changes tested?

Tested locally - the redirect works correctly, including with anchor
links (e.g., `upgrading.html#datafusion-46-0-0`).

## Are there any user-facing changes?

Yes - users clicking old changelog links will now be redirected to the
correct page instead of getting a 404.

Author: mishop-15

docs/source/conf.py

@@ -134,4 +134,5 @@
 redirects = {
     "library-user-guide/adding-udfs": "functions/index.html",
     "user-guide/runtime_configs": "configs.html",
+    "library-user-guide/upgrading": "/library-user-guide/upgrading/index.html",
 }