Change docs Primary content column from fixed size to responsive size

[FireballDWF]

Enable responsive design where primary content column can use full width of browser window, while now it's limited to a fixed size. Most noticeable when viewing the table at the end of
https://cloudcustodian.io/docs/aws/topics/securityhub.html#modes where user must manually use the horizontal scrollbar at the bottom of the table. On a sufficientble wide browser window, this would not be needed.