docs: add repository structure guide

[ming1523]

Summary

• add a standalone docs/repository-structure.md guide that maps the top-level repository layout
• group the main workspace members so contributors can quickly find binaries, service crates, shared libraries, tests, and tooling
• add a CONTRIBUTING.md entrypoint to the new guide for newcomers

Why

fuel-core already has substantial contributor documentation, but it does not yet provide a single repository-level map for people trying to understand where to start. A lightweight structure guide makes the codebase easier to approach without changing runtime behavior or duplicating implementation details.

Validation

• verified the documented directories and workspace groupings against the current repository tree and root Cargo.toml
• verified the new guide is linked from CONTRIBUTING.md
• docs-only change, no runtime behavior changed

This is a docs-only change. A no-changelog path seems appropriate if maintainers agree.

[cursor]

PR Summary

Low Risk
Docs-only changes with no impact to runtime behavior or build logic.

Overview
Adds a new docs/repository-structure.md guide that maps the repo’s top-level directories, workspace member groupings, and common starting points for contributor tasks.

Updates CONTRIBUTING.md to link newcomers to the new guide and fixes the trailing code block termination/newline at the end of the file.

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