Add `/llms.txt` file to site.

Impacted Sections

List UNTP specification page(s) that are impacted by this issue. For example:

• https://spec-untp-fbb45f.opensource.unicc.org/

Issue Description

The world is shifting from using traditional search engines (Google) to find information, but instead using LLMs (ChatGPT, etc) for information. The UNTP should have a /llms.txt file which provides structured information that LLMs can be trained on to better understand the UNTP.

I Have

• [n/a] Added the relevant labels (e.g. WG-Adoption, WG-Conformity, WG-SupplyChain, WG-Technical, WG-Steering, etc.).