Install RTD docs into RTD's expected venv path

Without UV_PROJECT_ENVIRONMENT, uv creates .venv/ inside the project
and RTD's sphinx step (which calls $READTHEDOCS_VIRTUALENV_PATH/bin/python
directly) fails with 'python: not found'. Point uv at RTD's venv so the
install lands where the runner looks for it.

Author: jeandet

.readthedocs.yaml

@@ -14,7 +14,7 @@ build:
       - asdf install uv latest
       - asdf global uv latest
     install:
-      - uv sync --group docs --frozen
+      - UV_PROJECT_ENVIRONMENT=$READTHEDOCS_VIRTUALENV_PATH uv sync --group docs --frozen
 
 sphinx:
   configuration: docs/conf.py