doc: add requirements.txt

Add a minimum sphinx version requirement for the docs; RTD needs this to
know what version of sphinx to install.

Signed-off-by: Quentin Young <qlyoung@nvidia.com>
(cherry picked from commit 826186eab217a590d86a7bdfaab6174da51c79b2)

diff --git a/doc/requirements.txt b/doc/requirements.txt
new file mode 100644 (file)
index 0000000..debc7f1
--- /dev/null
+++ b/doc/requirements.txt
@@ -0,0 +1 @@
+sphinx==4.0.2