Add migration link changelog

[paul-ohl]

Description

I added a link to the migration guides at the top of the changelog. Since the website did not have an index for the migration guides, I added it. And I also used that opportunity to fix the changelog tests which were broken due to username changes.

Fixes #3213

Checklist

• [X] I have reviewed my own code
• [X] I have added tests (Technically I fixed some tests)

[paul-ohl]

I'm sorry, it may be a bit of a stupid question, but is it possible to check the CI before pushing to remote?

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit b55aa96):

https://yew-rs--pr3471-add-migration-link-c-7s5dw056.web.app

_{(expires Wed, 25 Oct 2023 07:39:01 GMT)}

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