spec/intro/index.md: prettify with Mermaid diagrams

zero_gearrequested to merge
zero_gear/torspec:main into main

Checks performed:

  1. bin/build_html
  2. firefox html/intro/index.html

Screenshots that show renderings of diagrams Screenshot_from_2025-04-12_19-50-08

Screenshot_from_2025-04-12_19-50-18

Screenshot_from_2025-04-12_19-50-32

Edited by zero_gear

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