docs: simplify blockquote design for sharp corners

Remove full border and box-shadow from blockquotes, keeping only the
left border accent. This eliminates the diagonal corner artifacts that
occurred when mixing thin borders with thick left borders at 0 radius.

Changes:
- Remove 1px full border (only keep 3px left border)
- Remove box-shadow
- Increase background opacity slightly for better contrast
- Cleaner look that works well with sharp corners

Author: ammar-agent

docs/theme/custom.css

@@ -293,14 +293,11 @@ table code {
 
 /* Blockquotes with GitHub-style alerts support */
 blockquote {
-  border: 1px solid var(--color-border);
-  border-left: 4px solid var(--color-plan-mode);
-  background: color-mix(in srgb, var(--color-background-secondary), white 2%);
+  border-left: 3px solid var(--color-plan-mode);
+  background: color-mix(in srgb, var(--color-background-secondary), white 3%);
   padding: 1rem 1.2rem;
   margin: 1.5rem 0;
-  border-radius: var(--radius-md);
   color: var(--color-text);
-  box-shadow: var(--shadow-1);
 }
 
 blockquote p {