fix(docs): replace 3 dead links with working alternatives

[Ankur-Kataria]

What is this?

This is a docs-only change that replaces three dead links reported in issue #900.

Original URL	Status	Replacement
http://sahatyalkabov.com/create-a-character-voting-app-using-react-nodejs-mongodb-and-socketio	404	Wayback Machine snapshot (2023-02-05) — full article preserved
http://dev.stephendiehl.com/fun/	Timeout / unreachable	Official GitHub repo: https://github.com/sdiehl/write-you-a-haskell
http://thinkster.io (Resources section)	DNS resolution failed	Wayback Machine snapshot (2025-09-22)

All three replacement URLs return HTTP 200. python3 scripts/check_readme.py lint exits with errors=0 warnings=0.

Checklist

• This PR adds/fixes exactly one tutorial (or is a docs/tooling-only change).
• Free and open -- no paywall, login wall, or required newsletter signup.
• Project-based -- following it, the reader builds a complete, working artifact.
• I searched README.md for this URL and title -- it is not already listed.
• Entry uses - [Title](https://...) format, placed under the correct section (nested - [Part N](...) for multi-part series).
• No URL shorteners.
• I ran python3 scripts/check_readme.py lint locally and it exits 0.
• Relationship disclosure: no relationship to the author.

Fixes #900