docs: improve README clarity — explain what the tool does and why

[Copilot]

• Improve README.md to make it clearer what the tool does:
  • Add a "How it works" section with step-by-step explanation
  • Add a note pointing out that the banner at the top of the README is the actual output of this action
  • Show an example of the generated HTML that gets inserted into README.md
  • Improve Overview section to better explain the problem being solved
  • Apply review suggestion: rephrase heading example sentence ("for example, # My Project")
  • Apply review suggestion: simplify example URLs (use ... placeholder and {project_name} for alt)

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