docs: add getting started guide for new users

[jahrulezfrancis]

Description

This PR adds a beginner-friendly "Getting Started" guide to the documentation, as requested in issue #256. The guide is designed to take a new user from installation to their first debug session in under 10 minutes.

Key Changes

• [NEW] docs/getting-started.md: Implementation of a step-by-step walkthrough covering:
  • Installation via cargo install and binary downloads.
  • Building a sample Soroban contract to WASM.
  • Running basic debug commands (soroban-debug run).
  • Understanding results, budget tracking, and storage changes.
  • Introduction to interactive debugging mode.
• Readme.md: Added a direct link to the guide in the introduction to improve discoverability for new users.

Verification

• Verified all repository paths used in the guide (e.g., examples/sample_contract).
• Commands were double-checked for correctness against the current CLI interface.
• Checked for cross-browser accessibility and screen-reader compatibility where applicable.

Closes #256