docs: improve README clarity, structure, and integrator guidance

[Dark-Brain07]

Description

This PR improves the repository documentation to provide a clearer entry point for integrators and developers. It restructures the README.md to explain the core concepts of 0x Settler upfront, without changing any existing technical details or addresses.

Changes

• Introduction: Replaced the brief one-liner with a comprehensive definition of the protocol.
• New Sections:
  • What is Settler?: Explains the role of Settler in the 0x stack (Security, Efficiency, Flexibility).
  • Why Permit2?: Details the security and gas-efficiency benefits of using Uniswap's Permit2.
  • How it Works: Provides a high-level, 4-step workflow (Quote -> Sign -> Execute -> Settle) to help users visualize the process.
• Integration Guide: Clarified the best practices for interacting with the contracts, specifically regarding address resolution.

Motivation

To make the 0x Settler protocol more accessible to new integrators by providing better context and high-level explanations before diving into technical specifics.

[immunefi-magnus]

🛡️ Immunefi PR Reviews

We noticed that your project isn't set up for automatic code reviews. If you'd like this PR reviewed by the Immunefi team, you can request it manually using the link below:

🔗 Send this PR in for review

Once submitted, we'll take care of assigning a reviewer and follow up here.

[Dark-Brain07]

Summary

This PR improves documentation by fixing typos in the README and expanding internal comments to clarify intent.

Changes

• README.md: Fixed typos faciliate -> facilitate and negligble -> negligible.
• src/core/Basic.sol: Expanded TODO to clarify the purpose of the zero bps check.
• src/core/BalancerV3.sol: Clarified TODO regarding price limits for partial filling.

[Dark-Brain07]

Hi @dekz, the PR is ready for review!

Changes include:

• README improvements for integrator onboarding
• Typo fixes in README
• Clarified TODO comments in Basic.sol and BalancerV3.sol
• Added NatSpec to core contracts
• Added PR template and security policy

Let me know if you'd like any changes!