Update plugin documentation link to new devdocs URL

Changed the plugin documentation URL from the deprecated developer.mautic.org/#plugins to the new devdocs.mautic.org/en/5.x/plugins/getting_started.html

Fixes #556

Author: Lokesh9106

docs/links/plugins_documentation.py

@@ -2,6 +2,6 @@
 
 link_name = "Plugins developer documentation" 
 link_text = "Plugins developer documentation" 
-link_url = "https://developer.mautic.org/#plugins" 
+link_url = "https://devdocs.mautic.org/en/5.x/plugins/getting_started.html"
 
-link.xref_links.update({link_name: (link_text, link_url)})
+link.xref_links.update({link_name: (link_text, link_url)})