frontend/next: change all doc chatgpt.html links + feature page tweaks

[haraldschilly]

Description

• switch to the ai.html page for docs
• mention all enabled AI models on AI feature page
• some minor tweaks, like, accurately defining the "Customize" type and getting rid of a few too specific "ChatGPT" mentions.

Checklist:

• [ ] Testing instructions are provided, if not obvious
• [ ] Release instructions are provided, if not obvious