Documentation: move type system docs one level out

[maxitg]

Changes

• Since type system docs actually include more than just "symbols and functions" (e.g., the README there is essentially a guide page), this proposes to move them one level out, e.g., /Documentation/TypeSystem.

Comments

• No changes necessary to the links because type system docs use relative links, and the rest of the documentation does not refer to the type system yet.

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This change is