Update .readthedocs.yaml to install sphinx_rtd_theme

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: Id9de494a90a4616a9b42d210513d7cc278943647
diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index f89fc90..f11730e 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -14,6 +14,11 @@
     # nodejs: "19"
     # rust: "1.64"
     # golang: "1.19"
+#  apt_packages:
+#    - 
+  jobs:
+    post_create_environment:
+      - pip install sphinx_rtd_theme
 
 # Build documentation in the "docs/" directory with Sphinx
 sphinx: