style: 💄 add extra.scss with styles for page headers (#223)

# Description

The scss is simplified from
[tidyverse/tidytemplate](https://github.com/tidyverse/tidytemplate)
which is MIT licensed. In their `README` is does say "Please don’t use
it for your own package.", but since that refers to using the entire
template, I believe that using a simplified version of some of their
scss is ok. What do you think?

Needs a thorough review.

## Checklist

- [X] Ran `just run-all`

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: signekb

pkgdown/extra.scss

@@ -0,0 +1,29 @@
+// Simplified from https://github.com/tidyverse/tidytemplate
+
+.page-header {
+  border-bottom: none;
+  margin-bottom: 2rem
+}
+
+.template-home {
+  .page-header {
+    display: flex;
+    gap: 1rem;
+    align-items: center;
+    margin: 2rem 0;
+    padding: 2rem 0;
+    h1 {margin-top: 0rem}
+
+    @include media-breakpoint-down(sm) {
+      margin: 0.5rem 0;
+      padding: 0.5rem 0;
+    }
+  }
+  img.logo {
+    margin-left: 0;
+
+    @include media-breakpoint-down(sm) {
+      width: 100px;
+    }
+  }
+}