Add documentation for sxss knitr engine

[cderv]

At least small examples and links to where to look for more

---
output:
  html_document: default
---
```{scss}
$main-color: salmon;
.bright {
  background-color: $main-color;
}
```
This is a [*bright*]{.bright} word.

---
output:
  html_document: default
---
```{sass}
$main-color: salmon
.bright 
  background-color: $main-color;
```
This is a [*bright*]{.bright} word.

[cderv]

I just remember there was a chapter on the R Markdown Cookbook too. https://bookdown.org/yihui/rmarkdown-cookbook/eng-sass.html

It may worth transitioning all engine related doc into the Definitive Guide at the end - this seems like a better place.

Anyway, adding a link from this book to the cookbook is the least we need to do I think