Add Dualmark to the implementations list (TypeScript / Astro / Next.js / Cloudflare)

Hi — would Dualmark be welcome alongside the existing JavaScript Implementation in the spec's "Various tools and plugins" section?

[Dualmark](https://dualmark.dev) is an open-source implementation of llms.txt + markdown content negotiation that ships as installable packages for several JavaScript frameworks. Apache 2.0, npm provenance attested.

- **`@dualmark/core`** — framework-agnostic primitives (content negotiation, AI-bot detection, llms.txt renderer). Zero runtime deps, ESM+CJS, TypeScript.
- **`@dualmark/astro`** — Astro integration (auto-generates `.md` endpoints + `/llms.txt` from collection config)
- **`@dualmark/nextjs`** — Next.js App Router adapter
- **`@dualmark/cloudflare`** — Cloudflare Workers edge adapter
- **`@dualmark/cli`** — conformance runner (`dualmark verify `)
- **Spec extension:** [AEO Specification v1.0](https://github.com/dodopayments/dualmark/tree/main/spec) — RFC-2119 formatted, framework-agnostic; extends llms.txt with content negotiation, markdown-twin discovery, and an AI-bot UA registry

Live llms.txt: https://dualmark.dev/llms.txt

Suggested entry text for `nbs/index.qmd` (happy to send the PR if you like the framing):

> - [Dualmark](https://dualmark.dev) — TypeScript implementation of llms.txt + content negotiation, with adapters for Astro, Next.js, and Cloudflare Workers. Includes a conformance CLI (`dualmark verify `).

Drop a 👍 if I should send the PR, or let me know if you'd prefer different phrasing / a different section.

Edit nbs/index.qmd in the spec's "Various tools and plugins" section. Start by reviewing the existing JavaScript implementation entry, then add the proposed Dualmark wording; done means the implementation appears with its link and listed framework adapters.

Written by the indexing model from the issue text.