docs: link Rust tutorials to runnable source

[huklaa]

Advances the source-link portion of #111 by making runnable Rust examples easier to find from the docs.

Currently covers:

• Rust Client index → direct link to the complete rust-client/src/bin example directory
• Creating Accounts and Faucets → rust-client/src/bin/create_mint_consume_send.rs
• Interacting with Public Smart Contracts → rust-client/src/bin/counter_contract_increment.rs
• Delegated Proving → rust-client/src/bin/delegated_prover.rs
• Mappings in MASM → rust-client/src/bin/mapping_example.rs

This keeps the change documentation-only and leaves the broader client-tool migration out of scope.

Refs #111