docs: reduce main content max-width from 90% to 75%

Author: bnusunny

docs/guide/theme/custom.css

@@ -47,7 +47,7 @@ code, pre, .hljs {
 }
 
 .content main {
-    max-width: 90%;
+    max-width: 75%;
     line-height: 1.75;
 }