Doc: Add a DSL guide

[japgolly]

For instance, it's not very clear that there's a difference between style and styleS in inline styles.

[antonkulaga]

I thin the main problem of current docs is that they devote too much attention to inline styles forgetting standalone ones.

[WayneWang12]

Please at least tell me how to use standalone css in the application. I can't find the way in the whole document.