Dialog goes over sticky header in documentation

[leebyp]

As shown:

[llorca]

Not a problem, dialogs have a higher z-index than anything else on purpose. I do wonder if it's that useful to show a static dialog without its backdrop

[leebyp]

It's useful to see what the CSS API produces inline as a quick reference without needing to click the button in the JS API. The dialog should have a high z-index in general, but I don't think it should specifically for this purpose in the documentation, and we should override it. It just looks bad when you're scrolling through the docs.

[giladgray]

fix it ben https://github.com/palantir/blueprint/blob/master/packages/docs/src/styles/_sections.scss#L254-L264