fix(docs): prevent a virtual environment from failing a build

sphinx-build is passed a blanket "." to build all docs. However, if a
virtual environment is placed within the docs directory, sphinx will try
to build it which will fail due to some weird files it has.

This excludes the most common virtual environment directories from the
build to prevent this.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Ieeb14cfc5730d21c986611feb0ed379c58dfcae2
diff --git a/.gitignore b/.gitignore
index b005fab..1f4efb6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -43,3 +43,9 @@
 
 # Node.js
 node_modules/
+
+# common python virtual environment directories
+.env/
+env/
+.venv/
+venv/