Add 'Book' link to header and footer menus (#355)

* Add 'Book' link to header

* Update en.yml

* Update links.yml

data/links.yml

@@ -31,6 +31,8 @@ about_us:
 stay_up_to_date:
   - name: nav.blog
     url: /blog
+  - name: nav.book
+    url: https://link.springer.com/book/10.1007/978-3-031-50784-7
   - name: nav.press
     url: /press
   - name: nav.academy

locales/en.yml

@@ -356,6 +356,7 @@ en:
     academy: Academy
     association: Association
     blog: Blog
+    book: Book
     close_menu: Close menu
     code: Code
     community: Community

source/partials/header.html.erb

@@ -22,6 +22,7 @@
         <%= link_to t('nav.first-steps'), "#{t(:path)}/first-steps", class: "inline-block py-4 w-full md:w-auto" %>
         <%= link_to t('nav.features'), "#{t(:path)}/features", class: "inline-block py-4 w-full md:w-auto" %>
         <%= link_to t('nav.community'), "#{t(:path)}/community", class: "inline-block py-4 w-full md:w-auto" %>
+        <%= link_to t('nav.book'), "https://link.springer.com/book/10.1007/978-3-031-50784-7", target: "_blank", rel: "noopener noreferrer", class: "inline-block py-4 w-full md:w-auto" %>
         <%= link_to t('nav.about'), "#{t(:path)}/about", class: "inline-block py-4 w-full md:w-auto" %>
       </div>
       <%= link_to t('nav.donate'), "http://donate.decidim.org/", target: "_blank", rel: "noopener noreferrer", class: "button inline-block text-center py-4 w-full md:w-auto" %>