Add book icon to xrefs to other books

[ghost]

(This would save us from adding a little arrow there as in the older stylesheets.)

Don't do this before v1.79 with its corrected xref-to-suffix mode...

[janajaeger]

@tomschr if you have some more details on format (SVG?), size etc., I could provide you with a little something.

[tomschr]

@janajaeger Sure, some details that I think could be useful:

• Format: SVG for sure. 😉
  Maybe we could use a document icon (SUSE gallery) or a book icon (FontAwesome).

• Size
  It has to fit between the line(s). Here is an example of an external link (watch for the arrow up icon after the link):

  

• Source code
  The arrow up icon was made with CSS and SVG. You can find it here:
  

  suse-xsl/source-assets/styles2022/sass/custom/content-block.sass

  Lines 5 to 11 in 204bfa1

  	a.link::after
  	background-repeat: no-repeat
  	color: transparent
  	content: "�\2197"
  	text-decoration: none
  	background-size: contain
  	background-image: url('data:image/svg+xml;utf-8,<svg xmlns="http://www.w3.org/2000/svg" width="13" height="17" viewBox="0 0 13 17"> <rect fill="#{$c_jungle_url}" fill-opacity="0.4" y="10" x="1" height="6" width="6" /><path fill="#{$c_funky_darker_jungle_url}" d="M 4,5 V 6.9658466 H 8.4687653 L 2.75,12.617749 4.1562488,14 10,8.2866892 v 4.5768078 h 2 V 5 Z" /></svg>')

  
  Hope that helps.