Remove 'next page' link from lang intro page

[mit-mit]

This 'next page' link on the very first page under Language, Language Introduction, is very confusing. You'd think that it navigates you to the Variables content just below the top content, but it takes you entirely out of the Language Introduction and deep into a very detailed topic on variables. I think just the off-ramp links at the bottom at https://dart.dev/language#additional-resources are sufficient without the 'next page' links.

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