minor: Add favicon (#20000)

## Which issue does this PR close?

- N/A.

## Rationale for this change

There is no favicon currently set for the DataFusion site.

For example, when searching on Google:
<img width="228" height="139" alt="image"
src="https://github.com/user-attachments/assets/c0d2cd43-26fb-4715-a10c-0e60b7853a0a"
/>

When a tab is opened:
<img width="257" height="42" alt="image"
src="https://github.com/user-attachments/assets/4deb74b1-f29f-4bb5-a18b-d634e8ac0b16"
/>

Adding the logo makes it look nicer.


## What changes are included in this PR?

Added favicon.

## Are these changes tested?

Manual checks.

## Are there any user-facing changes?

No.

Author: nuno-faria

docs/source/_static/favicon.svg

@@ -117,6 +117,8 @@
     "**": ["docs-sidebar.html"],
 }
 
+html_favicon = '_static/favicon.svg'
+
 # tell myst_parser to auto-generate anchor links for headers h1, h2, h3
 myst_heading_anchors = 3