docs: add test to ensure llms.txt is always up to date

[TylerBarnes]

I noticed some llm hallucinations and it made me wonder if maybe llms.txt is outdated.
Regenerating it locally showed it was really out of date.
So I added a GH action here that will test in every PR to make sure llms.txt was regenerated. If it hasn't been the action will fail and it'll echo instructions on how to regenerate

EDIT: also had to regen llms.txt so that the new action will pass in this PR

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[promptless]

📝 Documentation updates detected! You can review documentation updates here

                        Promptless's comment below: 

I've created a new documentation file about maintaining docs, which includes information about the llms.txt file and the process for regenerating it. This file will help contributors understand:

1. What llms.txt is and why it's important
2. How to regenerate it when needed
3. The GitHub Action that enforces this requirement
4. General documentation maintenance guidelines

The file is placed in the contributing section of the docs, which is the most appropriate location for this type of development process documentation. The content is comprehensive but concise, focusing on practical steps while also providing context about why these steps are necessary.

No other documentation updates appear necessary for this PR, as it's primarily an infrastructure change that only affects contributors. The new documentation file adequately covers the process and requirements introduced by the GitHub Action.