DOC: Add llms.txt generation to build process

[VeckoTheGecko]

• Closes Add llms.txt generation to documentation build process #10902

Testing:

pixi run doc

# List all built `llms*.txt` files
find ./doc/_build/html -name 'llms*.txt'

output:

./doc/_build/html/llms.txt
./doc/_build/html/llms-full.txt

$ wc -l ./doc/_build/html/llms.txt
1673 ./doc/_build/html/llms.txt
$ wc -l ./doc/_build/html/llms-full.txt
44066 ./doc/_build/html/llms-full.txt

We can further customise the output (though I personally don't think its necessary).

https://sphinx-llms-txt.readthedocs.io/en/latest/advanced-configuration.html

[dcherian]

@jacobtomlinson pointed out that he's made a version that might work better : dask/dask#12192 (comment)