docs(secp256k1): document local build loading and submodule init

[odudex]

• Document git submodule update --recursive --init as a fallback when embit was cloned without --recursive.
• Show how to symlink the local make output into secp256k1-zkp/.libs/ so Python picks up the native backend, with a one-liner to verify ecdh/rangeproof_rewind are exposed.

Test plan

• Follow the README steps on Linux and confirm the verification command prints True True.
• Repeat on macOS with the .dylib suffix.

[Copilot]

Pull request overview

This PR improves the secp256k1 build documentation by adding guidance for initializing the secp256k1-zkp submodule after a non-recursive clone, and by documenting how to make a locally built libsecp256k1 discoverable by embit’s Python ctypes loader.

Changes:

• Add a fallback git submodule update --recursive --init command for existing clones.
• Document where make outputs the shared library artifact.
• Add steps to symlink the local build into secp256k1-zkp/.libs and a Python one-liner to verify optional symbols (ECDH + rangeproof rewind) are available.

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