docs: Update contributing commit guidelines link

[thehanslevi]

Summary

• Replace the archived AngularJS commit guidelines link in the Contributing section with the Conventional Commits spec.
• Align the README guidance with this repo's @commitlint/config-conventional setup.

Validation

• rg -n "Angular Git Commit Guidelines|Conventional Commits|commitlint|config-conventional|Commit your changes" README.md package.json commitlint.config.js CLAUDE.md
• node -e 'const fs=require("fs"); JSON.parse(fs.readFileSync("package.json","utf8")); const c=require("./commitlint.config.js"); if(!c.extends.includes("@commitlint/config-conventional")) process.exit(1); console.log("package.json parses and commitlint uses @commitlint/config-conventional")'
• curl -L --max-time 20 -o /dev/null -s -w '%{http_code}\n' https://www.conventionalcommits.org/ returned 200
• git diff --check

No runtime tests were run because this only updates a README reference link.