docs: add 30-second quick start and streamline README for easy onboarding

[GalaxySciTech]

Motivation

• Make first-run experience frictionless so a developer can install and get an address within seconds instead of reading long docs.
• Reduce initial surface area and cognitive load by surfacing only the minimal steps required to generate and store an identity and derive a wallet.

Description

• Rewrote README.md to add a prominent Quick Start (30 seconds) section with a minimal bootstrap snippet that uses WalletManager.storage = () -> new File("./keystore");, Identity.createIdentity(...), and identity.deriveWalletByMnemonics(...) so a user can copy-paste and get an address immediately.
• Added a short Core Features (Recommended Minimum) section that recommends shipping only identity/keystore, a single chain (ETH or BTC), and offline signing for first release.
• Simplified top navigation and deprioritized longer reference content (extended offline signing examples and contact info) to keep the first-use path concise.
• This is a documentation-only change and does not modify runtime code.

Testing

• Ran the test suite with ./gradlew test and the build completed successfully with all unit tests passing.

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Codex Task