docs: update README to reflect v4 mainnet deployment and improve structure

[IAM-CW]

Related Issue

There are no existing issues. This just addresses outdated documentation language in the README that was written during v4's development phase (June 2023) and hasn't been updated since mainnet deployment.

Related context:

• OpenZeppelin audit recommended creating SECURITY.md (PR OZ N16: Create SECURITY.md #774)
• Issue [Bug]: Incorrect Documentation #397 previously identified a documentation inaccuracy in the README

Description of changes

Updated the v4-core README to reflect the current state of the protocol following the January 2025 mainnet deployment across 10+ chains.

Changes:

• Removed outdated "early stages" and "draft code" language that no longer reflects v4's production status
• Added deployment context referencing the nine independent audits and $15.5M bug bounty program
• Added Security section cross-referencing SECURITY.md and the bug bounty program with an audit table
• Added Documentation section linking to docs.uniswap.org, v4-periphery, CONTRIBUTING.md, and SECURITY.md
• Updated whitepaper reference to remove "draft" qualifier
• Improved repository layout with cleaner code block formatting
• Preserved all technical content: architecture description, hook callbacks, Solidity examples, and licensing