[docs] Add singlemarkdown builder

[jdillard]

Summary of changes

• Add singlemarkdown builder and use for llms-full.txt
  • Depends on singlemarkdown builder liran-funaro/sphinx-markdown-builder#37 merging
• Turn off native llms-full.txt

docs preview: https://sphinx-llms-txt--51.org.readthedocs.build/en/51/llms-full.txt