docs: add tech preview webhook docs (#8889) (#8901)

(cherry picked from commit 2875658)

Co-authored-by: Brandon Morelli <[email protected]>

Author: mergify[bot]

docs/apm-mutating-webhook.asciidoc

@@ -0,0 +1,4 @@
+// Pulls the APM mutating webhook docs from here:
+// https://github.com/elastic/apm-mutating-webhook/blob/main/docs/apm-mutating-webhook.asciidoc
+
+include::{apm-webhook-repo-dir}/apm-mutating-webhook.asciidoc[]

docs/features.asciidoc

@@ -14,6 +14,7 @@
 * <<cross-cluster-search>>
 * <<span-compression>>
 * <<monitoring-aws-lambda>>
+* <<apm-mutating-admission-webhook>>
 
 include::./apm-data-security.asciidoc[]
 
@@ -32,3 +33,5 @@ include::./cross-cluster-search.asciidoc[]
 include::./span-compression.asciidoc[]
 
 include::./aws-lambda-extension.asciidoc[leveloffset=+2]
+
+include::./apm-mutating-webhook.asciidoc[leveloffset=+2]

docs/integrations-index.asciidoc

@@ -3,9 +3,10 @@ include::{asciidoc-dir}/../../shared/attributes.asciidoc[]
 include::./notices.asciidoc[]
 
 :apm-integration-docs:
-:apm-package-dir:    {docdir}/apm-package
-:obs-repo-dir:       {observability-docs-root}/docs/en
+:apm-package-dir:        {docdir}/apm-package
+:obs-repo-dir:           {observability-docs-root}/docs/en
 :apm-aws-repo-dir:       {apm-aws-lambda-root}/docs
+:apm-webhook-repo-dir:   {apm-mutating-webhook-root}/docs
 
 :github_repo_link: https://github.com/elastic/apm-server/blob/v{version}
 ifeval::["{version}" == "8.0.0"]