Add Platform Engineering IDP concept page and diagram to Hugo docs

[Copilot]

Adds a conceptual Platform Engineering IDP view to the Hugo documentation. The new page captures the intended self-service flow from platform templates through the IDP UI and repository context to automation-stage selection and generated outputs.

• New docs page

  • Adds hugo-docs/content/docs/platform-engineering/_index.md
  • Introduces a focused explanation of the IDP model in DevOps-OS terms: templates, guided self-service, repo/commit context, stage selection, and generated artifacts

• Embedded conceptual diagram

  • Adds an inline SVG diagram directly in the Hugo page
  • Models the requested flow explicitly:
    • Templates → IDP UI → Git repo / commit → Select templates → Select automation stages → Submit → Outputs
  • Keeps the diagram self-contained in docs without introducing additional static asset management

• Docs discoverability

  • Links the new page from both docs entry points:
    • hugo-docs/content/docs/_index.md
    • hugo-docs/content/_index.md

Example excerpt from the added page:

## Conceptual flow

Templates → IDP UI → Git repo / commit → Select templates → Select automation stages → Submit → Outputs