Add a real-world scan walkthrough to the docs

[sanmaxdev]

New visitors decide in seconds whether a tool is worth trying. A short walkthrough helps a lot.

What to do

• Add a "Walkthrough" section (in the README or a new docs/walkthrough.md) that runs sentineldeck scan <a domain you control or example.com> and explains: the grade, a couple of findings, and one copy-paste fix being applied.
• Keep it honest and passive; do not paste any live secrets or session output.

Acceptance

• A clear, skimmable walkthrough that renders well on GitHub.

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