feat: add SDLC Registry section

[dangrondahl]

Summary

• Migrates the full SDLC Registry from the Hugo-based kosli-sdlc site into Mintlify docs
• Adds 23 control pages (Build, Release, Runtime, Lifecycle phases) and 9 risk pages, each with prose, mitigating cross-references, and compliance reference accordions
• Adds background and governance content (Background, Governance Bottleneck, Controls Engineering)
• Introduces colored card grids with dark mode support via custom CSS (style.css)
• Uses Mintlify reusable snippets for all cards — single source of truth for controls and risks referenced across multiple pages
• Wires up full navigation in docs.json under the SDLC Registry product (Overview, Risks, Controls tabs)

Test plan

• mint dev — verify SDLC Registry product appears in navigation
• Check Overview tab: Overview page shows all risk and control cards; Background, Governance Bottleneck, Controls Engineering pages render correctly
• Check Risks tab: all 9 risk cards display with correct colors; each risk page shows mitigating controls grid
• Check Controls tab: all 23 control pages render with correct phase colors; each control page shows mitigates-risks grid
• Verify light and dark mode card colors
• Check all card links navigate to the correct detail pages

[dangrondahl]

This is not intended to be merged, but rather a discussion on how a controls/risks catalog could look like.

Another good thing would be to have this information available for docs agents.