infra: content gap analysis

[marc0olo]

Context

After the content migration sprints, a systematic comparison of all upstream sources against written docs is needed to find what's missing or outdated.

Steps

1. List all portal source pages in .sources/portal/docs/
2. Map each to its corresponding page in docs/ (use migration-plan.md as a starting point)
3. For pages that exist in portal but not in docs: evaluate if they belong (some portal content is not relevant for the rewrite)
4. For pages that exist in docs: spot-check for staleness against .sources/ upstream content
5. File individual issues for any meaningful gaps discovered
6. Update .docs-plan/migration-plan.md with gap findings

Output

A structured report in .docs-plan/ (or as a GitHub issue comment) listing gaps and their priority.