docs: add link checks and curation guardrails

[7etsuo]

Summary

• Add an automated Markdown link-check workflow with lychee, plus a focused ignore list for browser-only/bot-blocked URLs.
• Add Dependabot updates for GitHub Actions and a PR template with curation checks.
• Add a README tag legend, link-check badge, archived tags, and three curated tutorial additions.
• Replace a currently timing-out Princeton regex tutorial link with a verified Wayback snapshot.

Added resources

• Simple HTTP Web Server in C/C++ from Scratch
• Tonc - GBA Programming
• Writing a C Compiler

Test plan

• Ran git diff --cached --check.
• Parsed workflow and Dependabot YAML locally.
• Verified README internal anchors.
• Verified new/changed external URLs with curl using a browser-like user agent.