infra: content lifecycle strategy

[marc0olo]

Context

Define how docs stay fresh as upstream sources evolve. This covers change detection, sync automation, and freshness ownership.

Questions to answer

1. Upstream change detection: How do we know when .sources/ submodules have content that invalidates our docs? (Submodule bump PRs? Dependabot-style automation?)
2. Sync automation: Which pages should be auto-synced vs. hand-written? (Currently: Motoko pages synced, everything else hand-written)
3. Freshness ownership: Who owns keeping each section current? (Define in CODEOWNERS + section-level ownership docs)
4. Review cadence: How often should pages be spot-checked against upstream? (Propose a schedule)

Depends on

• Content gap analysis (infra: content gap analysis #118) — gap findings inform which areas need the most attention
• Validation scripts (infra: restore validation scripts #114) — automated staleness checks

Output

A decision recorded in .docs-plan/decisions.md and/or a new CONTRIBUTING.md section on content lifecycle.