Improve README with Quick Start, clearer onboarding, and beginner-friendly structure

**Description**
The current README is technically strong and well-documented, especially for experienced Go developers. However, it can be slightly overwhelming for first-time users or developers new to the DigitalOcean API or Go SDKs.

This issue proposes improving the README to make onboarding smoother while keeping all existing advanced documentation intact.

**Suggested Improvements**
- Add a “Quick Start” section at the top with a minimal working example (authenticate → make a simple API call).
- Clarify the Installation section by explicitly separating Go modules (recommended) and legacy GOPATH usage.
- Promote security best practices by encouraging the use of environment variables for API tokens instead of hard-coded strings.
- Provide brief context before longer examples, such as Droplet creation (regions, sizes, images).
- Label advanced sections clearly, such as pagination and retry configuration, to avoid overwhelming beginners.
- Add a “Common Use Cases” section (e.g., list droplets, delete droplet, manage SSH keys).

**Why This Matters**
- Improves developer experience for first-time users
- Reduces setup friction and confusion
- Makes the SDK more accessible without removing any advanced functionality
- Aligns the README with modern OSS documentation standards

_I’d be happy to submit a PR implementing these README improvements if the maintainers are open to it._
`hacktoberfest'25`, `Documentation`

Start by reviewing the repository README and its current installation and example sections. Restructure the onboarding flow around a Quick Start, Go modules versus GOPATH, token environment variables, contextual examples, common use cases, and clearly marked advanced topics. Done means the requested beginner guidance is present while the existing advanced documentation remains intact.

Written by the indexing model from the issue text.