docs: Add stub documents for all links to point somewhere

docs/lint.md

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+Linting
+=======

docs/performance.md

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+Performance
+===========

docs/sandbox.md

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+Sandboxing
+==========

docs/typecheck.md

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+Type checking
+=============
+
+Luau supports a gradual type system through the use of type annotations and type inference. One someone gets around to writing this, this will have documentation on the details of the type system and examples of common problems/how to deal with them.