docs: document string interpolation syntax and update code examples across docs and site

Add a new "Interpolation" section to the values documentation explaining the `%(expr)` syntax with examples of complex expressions and nested interpolation. Update all code samples in README.md, getting-started.markdown, and index.markdown to use the new interpolation syntax instead of string concatenation. Adjust syntax highlighting in style.scss by adding a new `.si` class for interpolation spans with distinct background color, and tweak existing color values for strings, numbers, and keywords to improve visual distinction. Add the `\%` escape sequence to the list of supported escape characters in values.markdown.

doc/site/style.scss

@@ -270,7 +270,7 @@ footer {
   span.c1, span.cm { color: mix($code-color, $code-bg, 60%); }
 
   // Keywords.
-  span.k, span.kd, span.kc, span.nb  { color: hsl(200, 60%, 50%); }
+  span.k, span.kd, span.kc, span.nb  { color: hsl(200, 70%, 50%); }
 
   // Names.
   span.vg { color: hsl(180, 70%, 35%); }
@@ -278,11 +278,14 @@ footer {
   span.vc { color: hsl(130, 60%, 40%); }
 
   // Numbers.
-  span.m, span.mi, span.mf { color: hsl(30, 80%, 45%); }
+  span.m, span.mi, span.mf { color: hsl(90, 40%, 50%); }
 
   // Strings.
-  span.s, span.s2  { color: hsl(40, 80%, 45%); }
-  span.se          { color: hsl(45, 80%, 50%); }
+  span.s, span.s2  { color: hsl(40, 90%, 40%); }
+  // Escape.
+  span.se          { color: hsl(50, 90%, 45%); }
+  // Interpolation.
+  span.si          { color: hsl(50, 90%, 40%); background: hsl(40, 90%, 95%); }
 
   // Operators and punctuation.
   span.o           { color: hsl(200, 40%, 50%); }