docs(introduction): write the Introduction section content

[Bugs5382]

Summary

Replace the stub Introduction pages with real, plain-language content: What is CryptOS?, Why it is built this way, and Project status and roadmap.

Acceptance criteria

• The three Introduction pages have complete prose (no stub placeholder line).
• Accurate to the shipped code on main; anything not built yet is clearly labeled roadmap.
• Consistent voice, emoji, and status badges with the rest of the site.
• npm run build stays green.

Tasks

• introduction/what-is-cryptos
• introduction/why
• introduction/status-roadmap

Notes

First content stage. Establishes the reading level and voice before the remaining sections.