[6.0][DOCS] Fix hard-coded mapping type doc link (elastic#33547)

* [DOCS] Fix hard-coded mapping type doc link (elastic#33319)

Relates to elastic/docs#2521

This PR fixes a hard-coded docmentation link that will cease to work when we move away from "master" URLS:

> 17:07:03 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/beats/devguide/6.0/metricset-details.html contains broken links to:
17:07:03 INFO:build_docs:   - en/elasticsearch/reference/master/mapping-types.html

NOTE: This fix needs to be backported all the way to 6.0

* [DOCS] Fix hard-coded mapping type doc link (elastic#33491)

Co-authored-by: Lisa Cawley <lcawley@elastic.co>

docs/devguide/metricset-details.asciidoc

@@ -106,7 +106,7 @@ include::../../metricbeat/module/mysql/_meta/fields.yml[]
 
 There is another `fields.yml` file under `module/{module}/{metricset}/_meta/fields.yml` that contains all fields retrieved
 by the metricset. As field types, each field must have a core data type
-https://www.elastic.co/guide/en/elasticsearch/reference/master/mapping-types.html#_core_datatypes[supported by elasticsearch]. Here's a very basic example that shows one group from the MySQL `status` metricset:
+{ref}/mapping-types.html#_core_datatypes[supported by elasticsearch]. Here's a very basic example that shows one group from the MySQL `status` metricset:
 
 [source,yaml]
 ----

metricbeat/module/elasticsearch/node/_meta/docs.asciidoc

@@ -1,6 +1,6 @@
 === Elasticsearch node metricset
 
 The `node` metricset interrogates the
-https://www.elastic.co/guide/en/elasticsearch/reference/master/cluster-nodes-info.html[Cluster API endpoint] of
+{ref}/cluster-nodes-info.html[Cluster API endpoint] of
 Elasticsearch to get cluster nodes information. This metricset only fetches the data from the `_local` node so it must
 run on each Elasticsearch node.

metricbeat/module/elasticsearch/node_stats/_meta/docs.asciidoc

@@ -1,7 +1,7 @@
 === Elasticsearch node_stats metricset
 
 The `node_stats` metricset interrogates the
-https://www.elastic.co/guide/en/elasticsearch/reference/master/cluster-nodes-stats.html[Cluster API endpoint] of
+{ref}/cluster-nodes-stats.html[Cluster API endpoint] of
 Elasticsearch to get the cluster nodes statistics. The data received is only for the local node so this Metricbeat has
 to be run on each Elasticsearch node.