Migration guide fix

[Lorak-mmk]

This PR fixes 2 problems:

• One broken link to new migration guide.
• Table not rendering correctly in migration guide. It looks like Scylla's documentation system doesn't support tables in Markdown.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[Lorak-mmk]

@dgarcia360 Regarding second point, are we doing something wrong or tables are really not supported? It is rendered correctly by both VSCode and mdbook, but not in Scylla docs

[piodul]

Sorry for the sloppiness on my side. The table looked correct in vscode/github, but AFAIK tables are a markdown extension, so I shouldn't take it for granted that sphinx supports it.

As for the incorrect file name in the link, it looks like something that should be caught by CI. I am surprised that mdbook (?) doesn't produce an error. Maybe it's possible to adjust the CLI flags to catch issues like that in the future.