[Feature] Add automatic llms.txt generation via mkdocs-llmstxt plugin

[aberoham]

Adds automated llms.txt generation directly into the docs build pipeline using the mkdocs-llmstxt plugin, complementing the excellent llms.txt generation tutorial from PR #8421

[chenmoneygithub]

@TomeHirata I haven't really played with llms.txt, do you think the benefits outweigh the increased complexity of the docs?

[TomeHirata]

I think it's a good addition to have llms.txt in our doc to increase the visibility of DSPy in the LLM searched contents

[idiom-bytes]

Great contribution @aberoham!

[aberoham]

Thanks for the feedback @TomeHirata -- note I also adjusted vercel.json, hopefully that works in the live deployed docs environment. (At least, that's what Vercel themselves do)