Document the deployment capability in the docs Capabilities area

[MariusStorhaug]

MSXOrg needs the deployment capability documented in the canonical Capabilities area so the contract — a change to managed resources is approved together with its effect, and the approved effect is exactly what deploys — has a durable, technology-agnostic home separate from any one repository's research branch.

Context and request

The deployment capability was drafted as spec-and-design docs on a research branch in MSXOrg/Platform (branch terraform-deployment-research), which is not a durable home: capability docs belong in MSXOrg/docs under Capabilities, beside release-management and the others, per the Documentation Model and its "it starts with a need" flow. This Task brings the deployment capability's spec, index, and first design into the docs site as its canonical source.

The single outcome: a reader or agent finds the deployment capability under Capabilities, with a technology-agnostic spec (approval covers the code change and its per-environment effect across the whole promotion path, approved as one decision; the approved plan is exactly what deploys; fail-closed on stale plans; no per-environment gates) and a first design, "Deploying Azure from GitHub".

Acceptance criteria: the Capabilities index lists Deployment; Capabilities/deployment/ holds index.md, spec.md, and designs/azure-from-github.md, each with front matter; the pages are registered in the site navigation; and the index and link-check scripts pass. Out of scope: implementing the Terraform workflows themselves (they live in MSXOrg/Platform) and additional designs (deploying Azure from Azure DevOps, deploying AWS from GitHub), each a later addition that never changes the spec.

Technical decisions

Place the files under src/docs/Capabilities/deployment/ following the release-management house style, keeping the designs/ subfolder so the technology-agnostic spec can carry multiple designs. Each design is named "deploying a service provider from a CI/CD platform" so its two axes are unambiguous. Convert the cross-repo absolute msxorg.github.io links to relative links. Keep index.md's hand-written Spec/Designs tables (no INDEX:START/END markers) because the auto-generator only lists direct files and subfolders that carry their own index.md, so a generated table would drop the subfolder design. Register the pages in src/zensical.toml under Capabilities.

Implementation plan

Delivered by one pull request:

• Add Capabilities/deployment/{index.md, spec.md, designs/azure-from-github.md} with front matter and relative links.
• Register the pages in src/zensical.toml navigation.
• Run Update-DocumentationIndex.ps1 and Test-DocumentationLink.ps1 and fix anything flagged.