fix(pages): add docs/index.html so root URL serves architecture diagram

[kogunlowo123]

Summary

• Pages source is main@/docs, build status built, but root URL citadel-cloud-management.github.io/terraform-gcp-vpc-network/ returns 404 because there is no docs/index.html — only docs/architecture.html.
• Adds a tiny docs/index.html that meta-refreshes to architecture.html, so the canonical Pages URL resolves while existing direct links to architecture.html keep working unchanged.

Test plan

• After merge + Pages rebuild, hit /terraform-gcp-vpc-network/ and confirm it lands on the architecture diagram
• Confirm /terraform-gcp-vpc-network/architecture.html still works (unchanged)