doc: some additional google analytics code

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/conf.py

@@ -218,6 +218,7 @@ html_static_path = ['static']
 def setup(app):
    app.add_stylesheet("acrn-custom.css")
    app.add_javascript("https://www.googletagmanager.com/gtag/js?id=UA-831873-64")
+   # note more GA tag manager calls are in acrn-custom.js
    app.add_javascript("acrn-custom.js")
 
 # Custom sidebar templates, must be a dictionary that maps document names