governance: add delegated administration use case and evolving platform note to org structure article

[Copilot]

Customers frequently cite team-level administrative autonomy and org-scoped secret isolation as drivers for multiple orgs—this case was missing from the article. Adds it alongside a forward-looking note on GitHub's active investment in multi-org management tooling.

Changes

• New section — "Delegated administration and team autonomy" under When to use multiple organizations: covers org-scoped permissions (org owners, Security Manager, CI/CD Admin, custom org roles, Actions policies, org-level secrets) and why granting these in a shared org creates both isolation leakage and a central-team bottleneck
• Trade-off table — two new rows: Team administrative autonomy and Org-level secret isolation
• Summary — added paragraph referencing GitHub's ongoing investment in multi-org tooling, with four specific public changelog citations (Aug 2025–Feb 2026) covering custom org roles, enterprise teams governance preview, scale limit increases, and org API unification

Notes for reviewers

• "CI/CD Admin" is used as a built-in role name per the problem statement; confirm this matches current product terminology and add a docs link if so
• The Summary's changelog citations are currently inline prose—consider reformatting as a short list for scannability
• Pre-existing front matter issue: compliance, internal-auditor, developer-lead personas and administration, billing, compliance areas are not in docs/taxonomies.md; out of scope here but worth tracking