docs: add FAQ.md answering common setup and contribution questions (#462)

[ironhood]

Description

Closes #462

This PR introduces a new FAQ.md file at the repository root to address the most common setup, contribution, and architectural questions that new developers have when evaluating StellarKit API.

The questions and answers were synthesized from our existing documentation (api-design.md, CONTRIBUTING.md, and README.md) to ensure all technical explanations and conventions remain consistent across the repository.

Changes Made

• Added FAQ.md at the repository root containing 8 detailed questions and answers:
  1. Why is the API needed when Horizon already exists?
  2. Can it be used on mainnet?
  3. Is an API key required?
  4. Why are some endpoints cached and others not?
  5. How are Stellar Wave points awarded?
  6. What is the standard response envelope?
  7. How are Stellar amounts and fees formatted?
  8. How does pagination work?
• Updated README.md: Added a link to FAQ.md under the existing Documentation section for easy discovery.

Acceptance Criteria Reached

• FAQ.md exists at the repo root.
• Contains at least eight questions and answers.
• Answers are written in plain, direct language and are consistent with existing docs.
• Linked from the README.md under the "Documentation" section.

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[ironhood]

Hi Maintainer, kindly review my PR

[ironhood]

Hi Maintainer, kindly review my PR