support custom label for back-to-top link

[xijinping0]

This PR resolves #2308 by adding a backToTopLabel field to the toc config.

I have confirmed in examples/docs that this change is working locally. The label is set to Go back to top please in the screenshot:

Thank you in advance for your review.

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Latest commit: cc18f3a

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Name	Status	Preview	Comments	Updated (UTC)
nextra-v2	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 21, 2024 7:33am

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Name	Status	Preview	Comments	Updated (UTC)
nextra	⬜️ Ignored (Inspect)	Visit Preview		May 21, 2024 7:33am

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[dimaMachina]

Sorry, I am not plan to add more features in v2, use v3 as base branch

[dimaMachina]

superseded by #3258