MINOR: [R][Docs] Fix package title text not showing (#46640)

### Rationale for this change

On the current docs, the title of the package doesn't show up. It would be good if it did. I tried various sanctioned things but none worked. I am chalking it up as something to do with the cosmo theme we're using and a limitation of pkgdown's customization ability.

![image](https://github.com/user-attachments/assets/ffbc2540-f11e-4e68-8ac6-2e6de4ce8b70)

### What changes are included in this PR?

pkgdown will now insert a hard-coded CSS style to make the package text show up in white.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

Authored-by: Bryce Mecum <[email protected]>
Signed-off-by: Bryce Mecum <[email protected]>

Author: amoeba

r/pkgdown/extra.css

@@ -1,6 +1,8 @@
 /* Section text in navbar */
 .navbar-dark, .navbar[data-bs-theme="dark"] {
   --bs-navbar-color: #d9d9d9;
+  --bs-navbar-brand-hover-color: #d9d9d9 !important;
+  --bs-navbar-brand-color: #d9d9d9 !important;
 }
 
 /* Version number in navbar */