Add attributes for various client library URLs

This will help referring to these books from other documentation

shared/attributes.asciidoc

@@ -59,6 +59,10 @@ Elastic-level pages
 :java-rest:            https://www.elastic.co/guide/en/elasticsearch/client/java-rest/{branch}
 :jsclient:             https://www.elastic.co/guide/en/elasticsearch/client/javascript-api/{branch}
 :jsclient-current:     https://www.elastic.co/guide/en/elasticsearch/client/javascript-api/current
+:es-ruby-client:       https://www.elastic.co/guide/en/elasticsearch/client/ruby-api/{branch}
+:es-dotnet-client:     https://www.elastic.co/guide/en/elasticsearch/client/net-api/{branch}
+:es-php-client:        https://www.elastic.co/guide/en/elasticsearch/client/php-api/{branch}
+:es-python-client:     https://www.elastic.co/guide/en/elasticsearch/client/python-api/{branch}
 :defguide:             https://www.elastic.co/guide/en/elasticsearch/guide/2.x
 :painless:             https://www.elastic.co/guide/en/elasticsearch/painless/{branch}
 :plugins:              https://www.elastic.co/guide/en/elasticsearch/plugins/{branch}
@@ -108,6 +112,7 @@ Elastic-level pages
 :enterprise-search-ref: https://www.elastic.co/guide/en/enterprise-search/{branch}
 :app-search-ref:       https://www.elastic.co/guide/en/app-search/{branch}
 :workplace-search-ref: https://www.elastic.co/guide/en/workplace-search/{branch}
+:enterprise-search-php-ref: https://www.elastic.co/guide/en/enterprise-search-clients/php/{branch}
 :enterprise-search-python-ref: https://www.elastic.co/guide/en/enterprise-search-clients/python/{branch}
 :enterprise-search-ruby-ref: https://www.elastic.co/guide/en/enterprise-search-clients/ruby/{branch}
 :elastic-maps-service: https://maps.elastic.co
@@ -144,7 +149,7 @@ Elastic Cloud
 Elasticsearch
 //////////
 
-:es-feature-flag: This feature is in development and not yet available for use. This documentation is provided for informational purposes only. 
+:es-feature-flag: This feature is in development and not yet available for use. This documentation is provided for informational purposes only.
 
 //////////
 Commonly referenced paths in commonly referenced repositories.