Fix broken links

Rakefile

@@ -16,6 +16,7 @@ task :test_without_rebuild do
       "https://dnomak.com/flexiblegs/",
       "https://incident57.com/codekit/",
       "https://daringfireball.net/projects/markdown/",
+      "#",
     ],
     assume_extension: true,
   ).run

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

@@ -78,6 +78,10 @@ included at the exact point where the `@import` was written. What's more, any
 mixins, functions, or variables that were defined before the `@import`
 (including from other `@import`s) are available in the imported stylesheet.
 
+[mixins]: mixin
+[functions]: function
+[variables]: ../variables
+
 <% heads_up do %>
   If the same stylesheet is imported more than once, it will be evaluated again
   each time. If it just defines functions and mixins, this usually isn't a big

source/documentation/js-api.html.md.erb

@@ -179,6 +179,7 @@ A string representation of the error. In [Node Sass][], this is more detailed
 than [`error.toString()`][] or [`error.message`][]. In [Dart Sass][], it
 provides the same information.
 
+[Dart Sass]: /dart-sass
 [`error.toString()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/toString
 [`error.message`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/message
 

source/documentation/values/booleans.html.md.erb

@@ -4,8 +4,8 @@ introduction: >
   Booleans are the logical values `true` and `false`. In addition their literal
   forms, booleans are returned by [equality](../operators/equality) and
   [relational](../operators/relational) operators, as well as many built-in
-  functions like [`comparable()`](../functions/map#comparable) and
-  [`map-has-key()`](../functions/math#map-has-key).
+  functions like [`comparable()`](../functions/math#comparable) and
+  [`map-has-key()`](../functions/map#map-has-key).
 ---
 
 <% example(autogen_css: false) do %>