docs: track react v14 migration work (#3010)

### 🎯 Goal

Keep the React v14 migration audit in source control while the release
is still moving, so follow-up breaking changes and doc rewrites can
continue from a fixed baseline.

### 🛠 Implementation details

- add `breaking-changes.md` as the confirmed v13 to v14 breaking-change
tracker
- add `docs-plan.md` as the page-by-page docs debt tracker and execution
plan
- record the currently audited SDK commit
(`6ea7a78e4184fce6066f7318f9ebd57a5ff1474a`) in both files so future
mining can start from `6ea7a78e..HEAD`
- include the confirmed docs fallout and migration-guide inputs gathered
during the audit
- no tests run; tracker/docs files only

### 🎨 UI Changes

N/A

Author: oliverlaz