Add `next-llms-txt` to Integrations Directory (Next.JS integration)

### Summary
I'd like to request the addition of **[`next-llms-txt`](https://www.npmjs.com/package/next-llms-txt)** to the Integrations section on [llmstxt.org](https://llmstxt.org/#directories).

### About `next-llms-txt`
`next-llms-txt` is an npm package that provides seamless integration for generating and serving llms.txt files in Next.js applications. It simplifies the process of making Next.js websites LLM-ready by automating the creation and serving of LLM-friendly documentation according to the llms.txt specification.

### Package Details
- **NPM Package:** https://www.npmjs.com/package/next-llms-txt
- **GitHub Repository:** https://github.com/bke-daniel/next-llms-txt
- **Type:** Next.js integration/plugin
- **Category:** Tool for generating LLM-friendly content

### Why It Should Be Featured
1. **Framework-Specific:** Tailored specifically for Next.js, one of the most popular React frameworks
2. **Developer-Friendly:** Enables developers to easily implement the llms.txt standard without manual setup
3. **Active Development:** The package is maintained and aligns with the growing adoption of the llms.txt specification
4. **Consistent with Existing Integrations:** Similar to other featured integrations like `docusaurus-plugin-llms` and `vitepress-plugin-llms`

### Suggested Format for Listing
```
- [`next-llms-txt`](https://www.npmjs.com/package/next-llms-txt): Next.js integration for automatically generating and serving LLM-friendly llms.txt files
```

Thank you for considering this addition to the community integrations directory!

Locate the source for the Integrations section on llmstxt.org and review the existing entries for docusaurus-plugin-llms and vitepress-plugin-llms. Add the suggested next-llms-txt listing with its npm link and description, then verify that it appears correctly in the integrations directory.

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