Use page-custom-canonical (#296)

Set a page-attribute `:page-custom-canonical:` to override the default
canonical URL generated by Antora.

Author: recrwplay

preview-src/tables.adoc

@@ -1,6 +1,7 @@
 = Tables
 :nofooter:
 :page-ogtitle: This page uses the page-ogtitle attribute to generate a custom title for SEO meta
+:page-custom-canonical: https://neo4j.com/docs/cypher-manual/current/clauses/where/
 
 .No striping
 // Alternative to stripes attributes is

src/partials/head-info.hbs

@@ -3,8 +3,14 @@
     <link rel="canonical" href="{{{this}}}">
     {{/with}}
     {{else}}
+    {{#if page.attributes.custom-canonical}}
+    {{#with page.attributes.custom-canonical}}
+    <link rel="canonical" href="{{{this}}}">
+    {{/with}}    
+    {{else}}
     <link rel="canonical" href="https://neo4j.com{{{ page.attributes.canonical-root }}}{{ page.url }}">
     {{/if}}
+    {{/if}}
 
     {{#unless (eq page.attributes.pagination undefined)}}
     {{#with page.previous}}