use .Sites.Default instead of .Sites.First

# Problem

The warning
```
INFO  deprecated: .Sites.First was deprecated in Hugo v0.127.0 and will be removed in a future release. Use .Sites.Default instead.
```

appears

# Solution

Follow the warning. Use `.Sites.Default`

layouts/partials/docs/brand.html

@@ -1,5 +1,5 @@
 <h2 class="book-brand">
-  <a class="flex align-center" href="{{ cond (not .Site.Home.File) .Sites.First.Home.RelPermalink .Site.Home.RelPermalink }}">
+  <a class="flex align-center" href="{{ cond (not .Site.Home.File) .Sites.Default.Home.RelPermalink .Site.Home.RelPermalink }}">
     {{- with .Site.Params.BookLogo -}}
     <img src="{{ . | relURL }}" alt="Logo" />
     {{- end -}}