verl documentations

Build the docs

# If you want to view auto-generated API docstring, please make sure verl is available in python path. For instance, install verl via:
# pip install .. -e[test]

# Install dependencies needed for building docs.
pip install -r requirements-docs.txt

# Build the docs.
make clean
make html

Open the docs with your browser

python -m http.server -d _build/html/

Launch your browser and navigate to http://localhost:8000 to view the documentation. Alternatively you could drag the file _build/html/index.html to your local browser and view directly.