Improve repository discoverability with a project structure overview

## Problem

As a new contributor, it was difficult to understand the repository structure and determine the purpose of each folder when getting started.

The repository contains several directories (e.g., APIs, clients, tests, examples, docs), but there is currently no high-level overview explaining what each folder is responsible for. This can make onboarding and navigation harder for new contributors.

## Proposed Enhancement

Add a repository structure section (tree overview) to the `README.md` or contributor documentation that briefly explains the purpose of major directories.

## For Example

```text
cluster-inventory-api/
├── apis/ # API definitions (ClusterProfile, PlacementDecision)
├── client/ # Generated clients, informers, and listers
├── config/ # CRD manifests and Kubernetes configuration
├── pkg/ # Shared utilities and access logic
├── plugins/ # Credential provider plugins
├── examples/ # Example controllers and integrations
├── test/ # Integration and conformance tests
├── docs/ # Documentation and adoption guides
└── hack/ # Scripts for build, verification, and code generation
```