docs: explain Bicep and Terraform feature parity

[Paulo Lacerda (placerda)]

Summary

• add a public, plain-English overview of the Bicep-to-Terraform parity initiative
• place the page after the Bicep and Terraform implementation documentation in MkDocs navigation
• explain initial feature equalization, continuous maintenance, evidence requirements, and human approval boundaries
• add lifecycle and detailed sequence diagrams using the site's existing Mermaid support
• link the landing page, both implementation repositories, and the authoritative Bicep process, ownership, and inventory documents

Validation

• python -m mkdocs build --strict

Scope

This PR changes documentation only. It does not change deployment behavior, merge or release automation, run terraform apply, publish the site, or claim completed runtime parity.