Specify docs requirements

Use `pip install -r docs/requirements.txt` (e.g., in a virtual environment) to install Python dependencies required for building the documentation.

Change-Id: Ied9bd505bd6fc92f8c04f526ebd3ccacec306371

docs/requirements.txt

diff --git a/docs/requirements.txt b/docs/requirements.txt
new file mode 100644
index 0000000..dd9a35d
--- /dev/null
+++ b/docs/requirements.txt
@@ -0,0 +1,2 @@
+sphinx-rtd-theme>=3.0.2
+sphinx>=8.1.3