[DOCS] Update links to use shared attributes (elastic/x-pack-elasticsearch#1855)

Original commit: elastic/x-pack-elasticsearch@b1553510b1

docs/en/index.asciidoc

@@ -5,6 +5,7 @@
 :es-repo-dir:           {docdir}/../../../../elasticsearch/docs
 :es-test-dir:           {docdir}/../../../../elasticsearch/docs/src/test
 :plugins-examples-dir:  {docdir}/../../../../elasticsearch/plugins/examples
+:docs-dir:              {docdir}/../../../../docs
 
 include::{es-repo-dir}/Versions.asciidoc[]
 

docs/en/rest-api/graph.asciidoc

@@ -4,7 +4,7 @@
 
 The Graph "explore" API is accessible via the
 `/_xpack/graph/_explore` endpoint.
-See {kibana}/graph-api-explore.html[Explore API].
+See {kibana-ref}/graph-api-explore.html[Explore API].
 
 //* <<graph-api-explore>>