docs: explain Contract-Driven Development for component interfaces

[mnriem]

Description

Fixes github#4609.

Add a self-contained Contract-Driven Development guide and link it from the SDD overview, monorepo guide, and documentation navigation.

The guide starts with interacting components and their externally observable obligations, not repository coordination. It covers defining missing contracts, one authoritative owner with consumer participation, direct references within a repository, pinned artifacts or synchronized copies across repositories, verification on both sides, and deliberate compatible evolution.

Includes standalone website/orders/inventory and control-plane/data-plane/integration-tests examples. No new CLI commands, core template changes, mandatory umbrella repository, dependency resolver, or release orchestrator.

Context and credit

• Related real-world request: github/spec-kit#4583.
• Control-plane/data-plane scenario: github/spec-kit#4465.
• Credit to @TongyiDai for the accepted answer and contract-evolution follow-up. The rollout example adapts those recommendations and retains name attribution in the guide. The guide explains initial contract definition before discussing evolution and does not depend on readers opening the discussion.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

The CLI and sample-project checks above are not applicable to this documentation-only change; the Python suite was not run.

Passed:

• npx --no-install markdownlint-cli2 docs/guides/contract-driven-development.md docs/guides/monorepo.md docs/concepts/sdd.md
• git diff --check upstream/main
• Local relative-link and anchor checks, navigation entry uniqueness, and DocFX content inclusion.
• Review against the existing plan/tasks command behavior and the issue scope, including the user's subsequent requirement that the site be self-contained.

The diff against upstream contains only four documentation files. A full DocFX build and rendered-site visual inspection were not performed.

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (fill in the disclosure below)

AI disclosure: GitHub Copilot using GPT-6 Astra (model ID: gpt-6-astra) drafted, revised, reviewed, checked, committed, and submitted this documentation on behalf of @mnriem in an interactive session. The user supplied the architectural direction and requested refinements; the documentation edits and checks were performed autonomously by the agent, without a claim of line-by-line human review. Reasoning-effort configuration is not exposed to the agent. AI involvement is documentation and PR-text generation only, not production code changes.