Move plain-CSS function documentation into at-rules/function

This isn't ideal, since these functions aren't defined with @function,
but there's not really a better place to put it. It certainly doesn't
make sense in the built-in module documentation anymore.

config.ru

@@ -25,6 +25,7 @@ use Rack::Rewrite do
   r301 '/documentation/Sass/Script/Functions.html', '/documentation/modules'
   r301 '/documentation/Sass/Script/Functions', '/documentation/modules'
   r301 %r{/documentation/(Sass.*)}, 'http://www.rubydoc.info/gems/sass/$1'
+  r301 '/documentation/functions/css', '/documentation/at-rules/function#plain-css-functions'
   r301 %r{/documentation/functions(.*)}, '/documentation/modules$1'
 
   r301 %r{/(.+)/$}, '/$1'

data/documentation.yml

@@ -50,7 +50,6 @@ toc:
   - Boolean: /documentation/operators/boolean
 - Built-In Modules: /documentation/modules
   :children:
-  - Plain CSS: /documentation/modules/css
   - Numbers: /documentation/modules/math
   - Strings: /documentation/modules/string
   - Colors: /documentation/modules/color

source/documentation/at-rules/function.html.md.erb

@@ -291,5 +291,43 @@ with [weird syntax][]).
 
 [core library]: ../modules
 [custom functions]: ../js-api#functions
-[plain CSS functions]: ../modules/css
+[plain CSS functions]: #plain-css-functions
 [weird syntax]: ../syntax/special-functions
+
+### Plain CSS Functions
+
+Any function call that's not either a user-defined or [built-in](../modules)
+function is compiled to a plain CSS function (unless it uses [Sass argument
+syntax](../at-rules/function#arguments)). The arguments will be compiled to CSS
+and included as-is in the function call. This ensures that Sass supports all CSS
+functions without needing to release new versions every time a new one is added.
+
+<% example(autogen_css: false) do %>
+  @debug var(--main-bg-color); // var(--main-bg-color)
+
+  $primary: #f2ece4;
+  $accent: #e1d7d2;
+  @debug radial-gradient($primary, $accent); // radial-gradient(#f2ece4, #e1d7d2)
+  ===
+  @debug var(--main-bg-color)  // var(--main-bg-color)
+
+  $primary: #f2ece4
+  $accent: #e1d7d2
+  @debug radial-gradient($primary, $accent)  // radial-gradient(#f2ece4, #e1d7d2)
+<% end %>
+
+<% heads_up do %>
+  Because any unknown function will be compiled to CSS, it's easy to miss when
+  you typo a function name. Consider running a [CSS linter][] on your
+  stylesheet's output to be notified when this happens!
+
+  [CSS linter]: https://stylelint.io/
+<% end %>
+
+<% fun_fact do %>
+  Some CSS functions, like `calc()` and `element()` have unusual syntax. Sass
+  [parses these functions specially][] as [unquoted strings][].
+
+  [parses these functions specially]: ../syntax/special-functions
+  [unquoted strings]: ../values/strings#unquoted
+<% end %>

source/documentation/modules/css.html.md.erb

@@ -1,41 +0,0 @@
----
-title: Plain CSS Functions
-introduction: >
-  Any function call that's not either a [built-in](../modules) or
-  [user-defined](../at-rules/function) function is compiled to a plain CSS
-  function (unless it uses [Sass argument
-  syntax](../at-rules/function#arguments)). The arguments will be compiled to
-  CSS and included as-is in the function call. This ensures that Sass supports
-  all CSS functions without needing to release new versions every time a new one
-  is added.
----
-
-<% example(autogen_css: false) do %>
-  @debug var(--main-bg-color); // var(--main-bg-color)
-
-  $primary: #f2ece4;
-  $accent: #e1d7d2;
-  @debug radial-gradient($primary, $accent); // radial-gradient(#f2ece4, #e1d7d2)
-  ===
-  @debug var(--main-bg-color)  // var(--main-bg-color)
-
-  $primary: #f2ece4
-  $accent: #e1d7d2
-  @debug radial-gradient($primary, $accent)  // radial-gradient(#f2ece4, #e1d7d2)
-<% end %>
-
-<% heads_up do %>
-  Because any unknown function will be compiled to CSS, it's easy to miss when
-  you typo a function name. Consider running a [CSS linter][] on your
-  stylesheet's output to be notified when this happens!
-
-  [CSS linter]: https://stylelint.io/
-<% end %>
-
-<% fun_fact do %>
-  Some CSS functions, like `calc()` and `element()` have unusual syntax. Sass
-  [parses these functions specially][] as [unquoted strings][].
-
-  [parses these functions specially]: ../syntax/special-functions
-  [unquoted strings]: ../values/strings#unquoted
-<% end %>