docs: Reorganize documentation for audience separation

Separates end-user docs (docs/) from contributor docs (dev-docs/).

Created:
- dev-docs/ with benchmarks, migration-tests, html-samples, screenshots

Moved:
- Benchmark reports from docs/Migration/Run*.md to dev-docs/benchmarks/
- Migration test reports from docs/migration-tests/ to dev-docs/
- HTML samples from docs/Migration/WebformsHtml/ to dev-docs/
- Screenshots from planning-docs/ to dev-docs/

Deleted:
- samples/Run7-12WingtipToys/ (old test run artifacts)
- docs/samples/ (unused)

Updated:
- mkdocs.yml: Removed internal reports from published nav
- samples/readme.md: Updated folder documentation

Requested by: Jeffrey T. Fritz

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: csharpfritz