Create update_mathlib.sh

scripts/update_mathlib.sh

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+# Update Mathlib and the Lean toolchain.
+
+# Download the latest lean-toolchain file from the Mathlib4 repository
+curl -L https://raw.githubusercontent.com/leanprover-community/mathlib4/master/lean-toolchain -o lean-toolchain
+
+# Update the Mathlib dependencies and ensure doc-gen is also updated
+# The `-R -Kenv=dev` flag ensures that the development environment is updated, including doc-gen
+lake -R -Kenv=dev update