Improve PyDo README structure and onboarding flow for first-time users

**Description**
The current README is comprehensive and technically detailed, covering quickstart usage, pagination, retries, testing, Docker workflows, known issues, and roadmap. However, the amount of information presented early in the README can feel overwhelming for first-time users who want to quickly validate that the client works.

This issue proposes improving the structure and readability of the README while preserving all existing content.

**Proposed Improvements**
1. Add a short “Quick Start” section at the very top
A minimal example that covers:
- Installing `pydo`
- Setting `DIGITALOCEAN_TOKEN`
- Making a simple API call (e.g., listing droplets or SSH keys)

2. Reorder sections for a smoother learning curve
Suggested order:
- Overview
- Prerequisites
- Installation
- Quick Start
- Common Examples
- Pagination & Retries (Advanced)
- Testing & Development
- Known Issues
- Roadmap

3. Clearly label advanced sections
Sections such as:
- Pagination
- Retries & Backoff
- Tests with Docker
- Known Issues
could be explicitly marked as _Advanced_ to reduce cognitive load for beginners.

4. Highlight security best practices
Explicitly recommend using environment variables for API tokens in all examples (which is already done in most places, but could be reinforced early).

**Why This Matters**
- Improves first-time developer experience
- Makes the README easier to scan and navigate
- Reduces friction for users evaluating PyDo for the first time
- Keeps advanced users fully supported without removing any detail

_I’d be happy to submit a PR reorganizing the README and adding a concise Quick Start section if this direction aligns with the maintainers’ goals._

`hacktoberfest'25`