Fix stale documentation links and redirect fallback text

Describe the issue

Two documentation entry points contain stale content on the current main revision:

• python/README.md links to ./migration_guide.md, but that file is not present in the checkout; the migration guide is published under the current documentation site.
• python/docs/redirects/redirect_template.html uses link to example.com as the fallback link text, so the root redirect page shows unrelated placeholder text even though its destination is correct.

What do you want to see in the doc? Update the README link to the published migration page and replace the placeholder fallback text with destination-neutral wording.

Additional context Verified against main at commit 027ecf0a379bcc1d09956d46d12d44a3ad9cee14. The live pages returned HTTP 200 on 2026-09-16; the root page visibly contains the placeholder text.

Link to the doc page: https://microsoft.github.io/autogen/stable/