Phase 4: TOC, redirects, deletes

[docs/csharp/toc.yml](docs/csharp/toc.yml): remove lines 205–206 (legacy tutorial) and 293–296 (legacy concepts); add 4 new entries inside the `Null safety` node from PR 8.
8. [.openpublishing.redirection.csharp.json](.openpublishing.redirection.csharp.json): add 3 new redirects (alphabetical insertion); repoint existing redirects on lines 3126, 5389, 5447 (currently target `/dotnet/csharp/nullable-migration-strategies`) to new fundamentals URL to avoid chains. Run `sort-redirects` skill.
9. Delete [docs/csharp/nullable-references.md](docs/csharp/nullable-references.md), [docs/csharp/nullable-migration-strategies.md](docs/csharp/nullable-migration-strategies.md), [docs/csharp/tutorials/nullable-reference-types.md](docs/csharp/tutorials/nullable-reference-types.md).

Author: BillWagner