Add unique title for search page

For improved accessibility, page titles should ideally be unique. This gives the
search page a unique title. It doesn't reflect the search terms, but it does at
least say "Search - Playbook - dxw" rather than defaulting just to "Playbook -
dxw"

@see https://dequeuniversity.com/checklists/web/page-title

src/search.html

@@ -1,4 +1,5 @@
 ---
+title: Search
 ---
 <h1 id="search-heading"></h1>