Sort books by date, add disclaimer (#6176)

* sort books primarily by date so that most up-to-date books are on top 
* simplify headers; all books are paid, no need to mention it
* improve flow on small screens
* add a disclaimer on top noting that books are not endorsed by the
listing -- feedback on wording welcome

CC @krisstern @NotMyFault

---------

Co-authored-by: Kevin Martens <99040580+kmartens27@users.noreply.github.com>
Co-authored-by: Alexander Brandes <brandes.alexander@web.de>

content/_partials/books.adoc

@@ -0,0 +1,6 @@
+[NOTE]
+====
+The books in this list are not endorsed by the Jenkins project.
+The list of books on this page is provided for informational purposes, sorted from newest to oldest publication date.
+Since older books on the list can contain outdated information, the link:/doc/[Jenkins documentation] can help fill in any gaps.
+====

content/books.html.haml

@@ -7,21 +7,33 @@ title: Jenkins Related Books
   .col-md-12 {
     margin-bottom: 2rem;
   }
-  #jenkins-related-books {
-    margin-bottom: 2rem;
+  .admonitionblock {
+    margin: 2rem 0;
+  }
+  .book-image {
+    max-height: 300px;
+    max-width: 300px;
+    margin: 8px;
   }
+  @media(min-width: 576px) {
+    .book-image {
+       float: right;
+    }
+  }
+
+=partial("books.adoc")
 
-- site.books.values.sort_by{ |book| [book['type'], book['title'], book['publication_year']] }.reverse.each_with_index do |book, index|
+- site.books.values.sort_by{ |book| [-book['publication_year'], book['title']] }.each_with_index do |book, index|
+  %h4
+    %span
+      - if book['url']
+        %a{:href => book['url'], :target => '_blank'}
+          = book['title']
+      - else
+        = book['title']
   - if book['image']
-    %img{:src => expand_link('images/' + book['image']), :style => 'max-height: 300px; max-width: 300px; float: right; margin: 5px;'}
+    %img.book-image{:src => expand_link('images/' + book['image'])}
   %div{:style => 'margin-bottom: 2rem'}
-    %h4
-      %span
-        - if book['url']
-          %a{:href => book['url'], :target => '_blank'}
-            = book['title']
-        - else
-          = book['title']
     - if book['author']
       %h5
         Authored by
@@ -33,19 +45,11 @@ title: Jenkins Related Books
     - if book['publisher']
       %h5
         Published by
-        = book['publisher']
-    - if book['publication_year']
-      %h5
-        Published in
-        = book['publication_year']
-    - if book['publication_year']
-      %h5
-        Type:
-        = book['type']
+        = "#{book['publisher']}, #{book['publication_year']}"
     - if book['description']
       %p
         = book['description']
     - if book['quote']
       %blockquote
         = book['quote']
-    %div{:style => 'clear: both;'}
+    %div{:style => 'clear: both;'}