Add support for llms-full.txt

[jdillard]

I think it would be great if a singlemarkdown builder was added so that a llms-full.txt file could be generated.

It might also help if there was a config option to set the ouput filename (llms-full.txt), but that could be worked around via deploy methods.

[liran-funaro]

Can you please provide more information? Providing an example input and output will be helpful.

[timhoffm]

I think this is out of scope for sphinx-markdown-builder. There's a dedicated extension for that https://github.com/jdillard/sphinx-llms-txt

[jdillard]

sphinx-llms-txt has it's limitations (as I'm sure using the singlemarkdown might have), so having both approaches available would be nice (and in scope)