More clearer document regarding custom paging example

### Issue

https://react-slick.neostack.com/docs/example/custom-paging

From the example, it provides class `slick-thumb` to dotsClass setting and I assume adding `slick-thumb` will make thumbnails look like the example, but it doesn't because `slick-thump` css definition doesn't exist in either `slick.css` or `slick-theme.css`, but instead in `docs.css` which is supposed to be use in the document only.

The example doesn't say anything about manually defining `slick-thumb` css, and it having `slick-` prefix also makes me assume that the css has been included in default slick css file.

### Expectation

Either the example need to explicitly state that you need to create css for `slick-thumb` class yourself, or include `slick-thumb` class from `docs.css` into default slick css file.