Add alert to typechecking docs page informing readers of luaus offical typechecking docs

[gaymeowing]

Changes

The typechecking docs page now refers readers to luaus official type checking docs for a more up to date reference. As robloxs docs site is very clearly lagging behind currently, this isn't intended to be a replacement for the typechecking docs page but rather a way to give readers more up to date info in case its still lagging behind in future.

Checks

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• This contribution was created in whole or in part by me, and I have the right to submit it under the terms of this repository's open source licenses.
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• To the best of my knowledge, all proposed changes are accurate.