fix(docs): Fix back to top button display (#4670)

# Description of Changes
- Fix issue with back to top button displays incorrectly
- Move the button to bottom left not to overlap with Ask AI

# Demo and screenshots
- After

<img width="1459" height="894" alt="image"
src="https://github.com/user-attachments/assets/75b4edda-dc54-493f-9274-0ca78f10fac7"
/>


https://github.com/user-attachments/assets/4cf7c3f4-51cc-4987-87db-b065ce4f7746

- Before

<img width="173" height="78" alt="image"
src="https://github.com/user-attachments/assets/052be945-fd2b-4276-a129-89c34a6a818b"
/>



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# API and ABI breaking changes

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corresponding GitHub label. -->

# Expected complexity level and risk
1

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# Testing

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reviewers to do,
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- [x] Back to top button displays correctly and works

Author: clockwork-tien

docs/src/css/custom.css

@@ -594,6 +594,22 @@ table .table-accent {
   color: var(--clockworklabs-color-green);
 }
 
+button.theme-back-to-top-button {
+  --ifm-menu-link-sublist-icon: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='16' height='16' viewBox='0 0 24 24'%3E%3Cpath fill='rgba(0,0,0,0.5)' d='M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z'/%3E%3C/svg%3E");
+  right: auto;
+  left: 1.8rem;
+  background-color: var(--clockworklabs-color-n6);
+  border: 1px solid var(--clockworklabs-color-n5);
+
+  &::after {
+    background-color: var(--clockworklabs-color-n1);
+  }
+
+  &:hover {
+    background-color: var(--clockworklabs-color-n5);
+  }
+}
+
 /* Make the page content left aligned */
 .container {
   margin: 0 16px 0;