docs: link pdpp.dev from README

[tnunamak]

Summary

• Add a prominent opening README link to https://pdpp.dev using the requested specification wording.
• Keep the existing spec-first README framing and avoid badges or marketing copy.

Prior art checked

• OpenTelemetry specification links its hosted latest release at opentelemetry.io in the first README prose block.
• SemVer links semver.org from the opening paragraph.
• OAuth and OpenID draft repos list editor or rendered draft copies immediately after the title.
• Sigstore architecture docs lead with canonical specification documents rather than a badge block.

Checks

• python3 /home/tnunamak/code/dotfiles/ai/skills/local/asd-ste100/scripts/ste_lint.py --mode descriptive <new prose>
• pnpm spec:check
• git diff --check

Assisted-by: AI