feat: add llms.txt to chisel-docs

[asanvaq]

This PR sets up sphinx-llm in the Chisel documentation build process to automatically generate both llms.txt and llms-full.txt files.

The changes are the following:

• Added the sphinx_llm.txt to the extensions in conf.py
• Included the required sphinx-llm config in conf.py
• Added sphinx-llm to requirements.txt‎
• Disabled the generation of the llms.txt content on a per-build basis as there’s no need to have Sphinx spend cycles (re)generating all the files while, for example, using sphinx-autobuild to check small edits in the docs. This was added to the Makefile build targets.
• Added extra spaces in the variable list so the new variable is aligned with the rest.

[cjdcordeiro]

looks pretty good to me. thanks. just a couple of questions and nits

[lczyk]

would love to see a pastebin of llms.txt generated from main. overall +1 for the feature though -- we'd be indirectly improving users experiance in all present and future llms. since its nvidia-driven standard i have no doubts it will at the very least become one of the standards if not the standard for crawler instructions for docs