change .Sites.First to .Sites.Default because of deprecation

layouts/partials/docs/brand.html

 <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 -}}