docs: refer to eslint 9 migration guide

[froesecom]

The differences between .eslintrc config formats and eslint.config.js config formats can be confusing. Eslint has a migration guide that is helpful for understanding the differences and may be helpful for other users of eslint-plugin-jest.
Thanks for your hard work!

[G-Rath]

I've got mixed feelings about this because we're already linking to the main doc which has the migration guide on the sidebar, and ideally I'd like to leave this up to ESLint as much as possible (e.g. if they decide to move the migration doc, that's their call).

I would say this PR would be better made to ESLint themselves - I think it would be useful for the main configuration files page we're already linking to to include a link to the migration guide at the top

[froesecom]

Yes, fair enough. I only put this PR up because the eslint-plugin-jest docs don't provide examples for the eslint.config.js file and I found it pretty confusing to translate between .eslintrc and eslint.config.js. Anyway, I'll close this. If you'd like me to help with providing examples for eslint.config.js I'm happy to do so.