Update Spark Configuration and Remove Testnet (DOC-240)

Created by: ihsraham

This PR updates the Spark Wallet documentation to reflect the deprecation of TESTNET and provides clear instructions on how to obtain test funds for REGTEST.

Changes

• Removed TESTNET: Removed all references to "Spark Testnet" and the TESTNET configuration option across configuration.md, usage.md, README.md, and api-reference.md.
• Added Faucet Links: Added visible callouts and links to the Lightspark Regtest Faucet in configuration.md, usage.md, and README.md to help developers easily fund their local testing environments.
• Cleanup: Updated code examples and BIP-44 derivation path documentation to list only MAINNET, SIGNET, and REGTEST.

Ticket

DOC-240

Verification

• Configuration: Verified configuration.md to ensure TESTNET is removed from values and examples, and the faucet link is present.
• Usage & README: Verified callouts for the Regtest Faucet appear in usage.md and README.md.
• API Reference: Verified api-reference.md to ensure TESTNET is removed from constructor parameters and type definitions.