Doc: Add Fleet and Agent release notes for 8.18.8 (#1864)

Author: karenzone

docs/en/ingest-management/release-notes/release-notes-8.18.asciidoc

@@ -14,6 +14,7 @@
 
 This section summarizes the changes in each release.
 
+* <<release-notes-8.18.8>>
 * <<release-notes-8.18.7>>
 * <<release-notes-8.18.6>>
 * <<release-notes-8.18.5>>
@@ -28,6 +29,33 @@ Also see:
 * {kibana-ref}/release-notes.html[{kib} release notes]
 * {beats-ref}/release-notes.html[{beats} release notes]
 
+// begin 8.18.8 relnotes
+
+[[release-notes-8.18.8]]
+== {fleet} and {agent} 8.18.8
+
+[discrete]
+[[features-enhancements-8.18.8]]
+=== New features and enhancements
+
+Elastic Agent::
+
+* Agent cleans up downloads directory and the new versioned home if upgrade fails. https://github.com/elastic/elastic-agent/pull/9386[#9386] https://github.com/elastic/elastic-agent/issues/5235[#5235]
+* When there is a disk space error during an upgrade, agent responds with clean insufficient disk space error message. https://github.com/elastic/elastic-agent/pull/9392[#9392] https://github.com/elastic/elastic-agent/issues/5235[#5235]
+
+[discrete]
+[[bug-fixes-8.18.8]]
+=== Bug fixes
+
+Elastic Agent::
+
+* Include aggregated agent status in HTTP liveness checks. https://github.com/elastic/elastic-agent/pull/9673[#9673] https://github.com/elastic/elastic-agent/issues/9576[#9576]
+* Reduce-default-telemetry-frequency. https://github.com/elastic/elastic-agent/pull/9987[#9987]
+* Fix stuck upgrade state by clearing coordinator overridden state after failed upgrade. https://github.com/elastic/elastic-agent/pull/9992[#9992]
+* Include components units status in HTTP liveness checks. https://github.com/elastic/elastic-agent/pull/10060[#10060] https://github.com/elastic/elastic-agent/issues/8047[#8047]
+
+// end 8.18.8 relnotes
+
 // begin 8.18.7 relnotes
 
 [[release-notes-8.18.7]]