[DOCS] Fix R pkgdown build process

[andrewherren]

Describe the update you'd like to see to the docs
The pkgdown 2.2.0 update now appears to build MD outputs alongside HTML outputs (for ease of ingestion by LLMs). This causes a conflict in the way that mkdocs-material builds the documentation which seems to prevent the stochtree pkgdown site from being hosted.

This seems to be solvable by adding this line to _pkgdown.yml:

llm-docs: false