[Docs] Fix `site_url` for RunLLM (#21564)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

mkdocs.yaml

 site_name: vLLM
-site_url: https://docs.vllm.ai
+site_url: !ENV READTHEDOCS_CANONICAL_URL
 repo_url: https://github.com/vllm-project/vllm
 edit_uri: edit/main/docs/
 exclude_docs: |