Added redirect for gbfs.org/reference

... to https://gbfs.org/specification/reference/

mkdocs.yml

@@ -84,6 +84,7 @@ plugins:
         'tools-services.md': 'toolbox/index.md'
         'faq.md': 'learn/faq.md'
         'about.md': 'participate.md'
+        'reference.md': 'specification/reference.md'
   - external-markdown
   - i18n:
       docs_structure: folder
@@ -141,4 +142,4 @@ nav:
     - Guide: learn/guide.md
     - Data Policy: learn/data-policy.md
     - FAQ: learn/faq.md
-  - Participate: participate.md
+  - Participate: participate.md