Doc: Add shared attribute for messaging ecs default info (#13265)

Backports #13083 to 7.x
Co-authored-by: Ry Biesemeyer <yaauie@users.noreply.github.com>

docs/include/attributes-ls.asciidoc

@@ -0,0 +1,10 @@
+/////
+These settings control attributes for Logstash core content 
+in the Logstash Reference (LSR) only.
+ 
+Shared attributes for the plugin docs (in the LSR and VPR) should
+go in /docs/include/attributes-lsplugins.asciidoc instead 
+with a corresponding change to the VPR settings in 
+logstash-docs/docs/versioned-plugins/include/attributes-ls-vpr.asciidoc
+/////
+

docs/include/attributes-lsplugins.asciidoc

@@ -0,0 +1,12 @@
+/////
+These settings control attributes in the LSR only. 
+They correspond to the VPR settings in logstash-docs/docs/versioned-plugins/include/attributes-ls-vpr.asciidoc
+When we update one, we must update settings in the other location,
+
+Attribute text formatted without hard wrap is deliberate. 
+Otherwise, text breaks at return and content isn't displayed in its entirety. 
+/////
+
+
+:ecs-default: The `pipeline.ecs_compatibility` setting is available in {logstash-ref}/logstash-settings-file.html[`logstash.yml`] and `pipelines.yml`. Any value in `pipeline.ecs_compatibility` other than `disabled` is considered BETA and unsupported until Logstash 8.0 and the final 7.x version are released. Having this pipeline level flag set will cause even patch-level upgrades to automatically consume breaking changes in the upgraded plugins, changing the shape of data the plugin produces.
+

docs/index.asciidoc

@@ -3,6 +3,8 @@
 
 include::{docs-root}/shared/versions/stack/{source_branch}.asciidoc[]
 include::{docs-root}/shared/attributes.asciidoc[]
+include::./include/attributes-ls.asciidoc[]
+include::./include/attributes-lsplugins.asciidoc[]
 
 :include-xpack:         true
 :lang:                  en