message: add priority and `all.html` to sitemap
diff: <url>
  <loc>https://docs.rs/{{ release.crate_name }}/latest/{{ release.target_name }}</loc>
  <lastmod>{{ release.last_modified | escape_xml }}</lastmod>
  + <priority>1.0</priority>
  + </url>
  + <url>
  + <loc>https://docs.rs/{{ release.crate_name }}/latest/{{ release.target_name }}/all.html</loc>
  + <lastmod>{{ release.last_modified | escape_xml }}</lastmod>
  + <priority>0.8</priority>
  </url>
  {%- endfor %}
  </urlset>
  