Add RAG feature page

[1dom]

Easy approach for other to expand the docs: pick a feature or 2 from the intro page, and create a more in depth feature page for them.

[tjbck]

Thanks for the PR! We might want to keep the intro as-is, and put the rag.md file into our tutorial directory.

[1dom]

Thanks for open webui and the docs so far! Sorry, I should have been a bit more descriptive of my intent in the original comment:

1. this change was motivated by the intro page: I felt disappointed being a new user and reading those features, and then actually trying to figure out how to use remote content RAG. This is because the intro reads like it's largely AI generated and heavily oversells and under-explains the features. It reads like sales pitch when it's actually the only documentation for most of the features mentioned.
2. This is a description of the feature and how it works, not a tutorial on how to achieve a particular task.

I think for this to be a tutorial and fit with the rest, it needs to be restructured as a step-by-step example. Even then, the intro section on "Web Browser Capability" needs rewording or removing.

I'm happy to restructure as a tutorial and remove any change to the intro if that's your direction, but I think this approach feels like a nicer bridge between the hardcore sales intro and the hardcore "next do this" of the tutorials.

Thanks again for your work!

[tjbck]

Thanks!