Add llms.txt file for Nest

**As we know that Large language models increasingly rely on website information, but face a critical limitation: context windows are too small to handle most websites in their entirety.**
As nest project will be used by many other people and now it's LLM's. Having a llms.txt file will be a beneficial thing to do as it will serve as a single piece of file containing all the important information about the project for a concise and accessible way to help LLM's get trained / crawl out the necessary details which will definitely help bring more contributors and give more context to LLM's about this project.

**I don't think we have a llms.txt file. I would love to create it.**
A llms.txt file is just a markdown file will necessary information about the project.
The way it is implemented is by having a /llms.txt route that returns the markdown.

**My plan for implementation**
- create a folder called llms.txt (app router way of next js)
- add a new file called route.ts
- this file will be a GET route
- which will contain content written in a markdown style
- finally return the response (content) with headers content type text/plain

**Are you going to work on implementing this?**

- [X] Yes
- [ ] No

**Additional context**
[more about llms.txt](https://llmstxt.org/)
[llms.txt directory](https://llmstxt.site/)

Start by inspecting the existing Next.js app-router structure and determine where the proposed llms.txt/route.ts entry point belongs. Add the GET route with the project information in markdown-style text, then verify that /llms.txt returns the content with a text/plain response.

Written by the indexing model from the issue text.