Fix broken docs build due to non-ASCII character

Seen in GCB:

```
Step #2:   Conversion error: Jekyll::Converters::Scss encountered an error while converting 'css/main.scss':
Step #2:                     Invalid US-ASCII character "\xC2" on line 68
Step #2: jekyll 3.8.7 | Error:  Invalid US-ASCII character "\xC2" on line 68
```

This should fix it.

Closes bazelbuild#12187.

PiperOrigin-RevId: 335005755

site/_sass/be.scss

@@ -1,3 +1,5 @@
+@charset "utf-8";
+
 // Build Encyclopedia and Starlark Library
 
 pre.rule-signature {
@@ -65,7 +67,7 @@ ul.hlist li {
 }
 
 ul.hlist li:after {
-  content: " · ";
+  content: ' · ';
   margin: 0 1ex;
   font-weight: bold;
 }
@@ -104,7 +106,7 @@ $toc-color: #757575;
 
 // Command-line Reference
 dt {
-  margin-top: .5em;
+  margin-top: 0.5em;
 }
 
 dd {