docs: add RTD file (#369)

Copied from dpgen.

See https://blog.readthedocs.com/migrate-configuration-v2/

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

.readthedocs.yaml

@@ -0,0 +1,24 @@
+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Set the version of Python and other tools you might need
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.10"
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+   configuration: doc/conf.py
+
+# If using Sphinx, optionally build your docs in additional formats such as PDF
+formats: all
+
+# Optionally declare the Python requirements required to build your docs
+python:
+   install:
+   - requirements: doc/requirements.txt