docs: add B512 type documentation

[airope]

Closes #3241

Adds comprehensive documentation for the B512 type including:

• Overview and use cases (signatures, public keys)
• Code examples with region markers
• Creating and converting B512 values
• Contract integration examples

Follows the same pattern as the B256 documentation.

[cursor]

PR Summary

Low Risk
Low risk documentation-only change; no runtime or protocol logic is modified. Main risk is broken snippet includes or minor inaccuracies in examples.

Overview
Updates the B512 docs to clearly define its size/representation and highlight common use cases (signatures, public keys), and reorganizes the guide around SDK usage, contract interaction, and Sway usage.

Adds a new Sway snippet file (sway-b512-examples.sw) demonstrating creating a B512 from two b256 halves, passing it through a function, and using it with ec_recover_address.

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