docs(calm): document architecture timelines (schema, CLI, Hub UI)

[rocketstack-matt]

Feature Proposal

Target Project:

docs/ (Docusaurus site), with cross-links from cli/ and calm-hub/ READMEs where relevant.

Description of Feature:

PR #2496 introduced first-class architecture timelines end-to-end:

• a stored Timeline resource in calm-hub, with both explicit (authored) and implied (projected from semver) modes;
• calm timeline and calm diff --timeline CLI commands (with --from / --to for arbitrary moment pairs);
• a collapsible timeline bar in the calm-hub-ui diagram view (single + compare modes).

No user-facing documentation was added with that PR. This issue tracks landing the docs.

User Stories:

• As an architect, I want to read what a "timeline" is and how to author one, so I can model how my architecture evolves over time.
• As a CLI user, I want documented examples of calm timeline (generating an implied timeline from versioned architectures) and calm diff --timeline (diffing adjacent or arbitrary moments), so I can integrate the commands into CI.
• As a Hub user, I want to understand the timeline bar in the diagram view — single vs compare mode; the meaning of NOW / CURRENT / FROM / TO badges; the implicit-exit-compare behaviour; the difference between explicit and implied timelines; and that the diagram viewport persists across moment switches.

Current Limitations:

The only timeline references in docs/ today are the existing schema reference. Everything else about how to use the feature lives in code, tests, and the PR description for #2496.

Proposed Implementation:

• Schema reference under docs/ describing calm-timeline.json: moments, current-moment, details.detailed-architecture, ADR refs, the explicit-vs-implied distinction.
• CLI reference entries for calm timeline and calm diff --timeline (--from, --to, --format summary|json), with a worked example using the fixture in cli/test_fixtures/timeline-diff/.
• Hub UI how-to: the collapsible timeline bar, single vs compare modes, badges, the timeline-demo seed namespace as a try-it-yourself walkthrough.

Alternatives Considered:

Inlining all of the above into the PR that ships the feature was rejected to keep #2496 scoped to implementation review; a separate docs PR is easier to review and to land without blocking the feature.

Testing Strategy:

N/A (docs only); validate by reading the rendered Docusaurus pages and walking through the CLI / Hub examples end-to-end.

Documentation Requirements:

This issue is the documentation requirement; the checklist below tracks page-level deliverables.

Implementation Checklist:

• Timeline schema reference page under docs/
• calm timeline and calm diff --timeline entries in the CLI command reference
• Hub UI page covering the timeline bar (single / compare / persistence / badges)
• Walkthrough using the timeline-demo seed namespace

Additional Context:

Follow-up to #2496 (intentionally deferred to keep that PR scoped to implementation).