Fix duplicate import of color_schemes (#1173)

This is a deceptively simple PR that stops the double import of `color_schemes`. With @pdmosses' stellar suggestion, it's a simple two-liner!

## interaction with #1166

This is a clean merge!

## path forward for default syntax highlighting

However, this leaves an interesting question: if the user doesn't provide syntax highlighting as part of their color scheme, should we include a default set (in this case, the light theme)?

Broadly, I see a few arguments:

1. if we don't provide defaults, we'll break color schemes that don't define their own syntax highlighting
2. if we do provide defaults, we could unnecessarily bloat the file size

I think 1 is more pernicious than 2. Thus, my suggested path forward is:

- for now, merge this
- in `v1.0`, separate these concerns properly, and force each color scheme to provide its own syntax highlighting CSS. Provide a default file for users to import with `@import` or `@use`[^1]. 

[^1]: Separately, we're using `!default` wrong; [looking at the docs](https://sass-lang.com/documentation/variables#default-values), we need to be using it with `@use` for the defaults to take effect. Since we're not doing that, `!default` isn't actually doing anything! This is why variable overrides aren't propagating the way they should be (and thus, users need to do a lot of duplication). Fixing this is probably a v1 item, though I'll have to think about it more.

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

@@ -4,7 +4,9 @@ $logo: "{{ site.logo | relative_url }}";
 @import "./support/support";
 @import "./custom/setup";
 @import "./color_schemes/light";
+{% unless include.color_scheme == "light" %}
 @import "./color_schemes/{{ include.color_scheme }}";
+{% endunless %}
 @import "./modules";
 {% include css/callouts.scss.liquid color_scheme = include.color_scheme %}
 {% include css/custom.scss.liquid %}