Add new _sass/custom/setup.scss for variable definition (#1135)

This is an alternative PR that resolves #1011. Unlike #1013, this PR defines a *new* SASS file, `_sass/custom/setup.scss`, specifically designed for new custom variables (and other SASS-only constructs). It's imported after our `support` SASS files are (functions, variables), but otherwise is imported before all other files (ex, when CSS is emitted).

So, custom callout colors can now be defined in this file. I also move the custom callout colors present in `custom.scss` to the right location.

I've added some docs that briefly explain how to use the feature. Feedback is welcome!

---

As an aside, I chose not to add a `_includes/css` file that imports this, and then import that file. I think that's only necessary if we're trying to render liquid somehow in the SASS file; since we're not trying to do that for `setup.scss`, I've opted to not include it. If we think this is relevant, I can re-add it.

Co-authored-by: Peter Mosses <18308236+pdmosses@users.noreply.github.com>

_includes/css/just-the-docs.scss.liquid

@@ -2,6 +2,7 @@
 $logo: "{{ site.logo | relative_url }}";
 {% endif %}
 @import "./support/support";
+@import "./custom/setup";
 @import "./color_schemes/light";
 @import "./color_schemes/{{ include.color_scheme }}";
 @import "./modules";