Docs: add an entry for Sass processing in the main Content section

docs/content/documentation/content/sass.md

@@ -0,0 +1,42 @@
++++
+title = "Sass"
+weight = 110
++++
+
+Sass is a popular CSS extension language that approaches some of the harder
+parts of maintaining large sets of CSS rules. If you're curious about what Sass
+is and why it might be useful for styling your static site, the following links
+may be of interest:
+
+* The [official Sass website](http://sass-lang.com/)
+* [Why Sass?](https://alistapart.com/article/why-sass), by Dan Cederholm
+
+## Using Sass in Gutenberg
+
+Gutenberg processes any files with the `sass` or `scss` extensions in the `sass`
+folder, and places the processed output into a `css` file with the same folder
+structure and base name into the `public` folder:
+
+```bash
+.
+└── sass
+    ├── style.scss // -> ./public/style.css
+    ├── indented_style.sass // -> ./public/indented_style.css
+    ├── _include.scss // This file won't get put into the `public` folder, but other files can @import it.
+    ├── assets
+    │   ├── fancy.scss // -> ./public/assets/fancy.css
+    │   ├── same_name.scss // -> ./public/assets/same_name.css
+    │   ├── same_name.sass // CONFLICT! This has the same base name as the file above, so Gutenberg will return an error.
+    │   └── _common_mixins.scss // This file won't get put into the `public` folder, but other files can @import it.
+    └── secret-side-project
+        └── style.scss // -> ./public/secret-side-project/fancy.css
+```
+
+Files with a leading underscore in the name are not placed into the `public`
+folder, but can still be used as `@import` dependencies. For more information, see the "Partials" section of
+[Sass Basics](https://sass-lang.com/guide#partials).
+
+Files with the `scss` extension use ["Sassy CSS" syntax](http://sass-lang.com/documentation/#Formatting),
+while files with the `sass` extension use the ["indented" syntax](http://sass-lang.com/documentation/file.INDENTED_SYNTAX.html).
+Gutenberg will return an error if a `scss` and `sass` file exist with the same
+base name in the same folder to avoid confusion -- see the example above.