Add Independent Agent Release doc (#2824)

Relates elastic/kibana#228494

We want to provide information about the Independent Agent Release (IAR)
process to users.

This PR adds:
- a new "Elastic Agent release process" page with a section about IAR;
this page is a child of [Fleet and Elastic Agent / Install Elastic
Agents](https://www.elastic.co/docs/reference/fleet/install-elastic-agents)
- A mention of IAR versions in [Fleet and Elastic Agent / Manage Elastic
Agents in Fleet / Upgrade Fleet-managed Elastic
Agents](https://www.elastic.co/docs/reference/fleet/upgrade-elastic-agent)

### Screenshots

New "Elastic Agent release process" page:
<img width="1524" height="835" alt="Screenshot 2025-09-08 at 10 59 43"
src="https://github.com/user-attachments/assets/3dcd452c-e4f7-4cd1-bed4-a6383f8f6ed6"
/>

Edit to "Fleet and Elastic Agent / Manage Elastic Agents in Fleet /
Upgrade Fleet-managed Elastic Agents":
<img width="1524" height="909" alt="Screenshot 2025-09-08 at 11 09 46"
src="https://github.com/user-attachments/assets/68f2120a-e7e9-44db-b321-62666958d393"
/>

Author: jillguyonnet

reference/fleet/fleet-agent-release-process.md

@@ -0,0 +1,20 @@
+---
+navigation_title: "{{agent}} release process"
+applies_to:
+  stack: ga
+products:
+  - id: fleet
+  - id: elastic-agent
+---
+
+# {{agent}} release process [fleet-agent-release-process]
+
+## Scheduled releases
+
+{{agent}} follows a release process aligned with the broader {{stack}} release schedule. The latest features, enhancements, and fixes are documented in the [release notes](../../release-notes/fleet-elastic-agent/index.md).
+
+## Independent {{agent}} releases [independent-agent-releases]
+
+Independent {{agent}} releases deliver critical fixes and updates for {{agent}} and {{elastic-defend}}, independently of the full stack release. This is a more conservative process than a typical patch release, and only modifies the specific {{agent}} components needed for a targeted fix. For example, an independent {{agent}} hotfix release for {{elastic-defend}} would only change the endpoint-security executable, with the remaining executables being exactly those released in the previous patch.
+
+In independent {{agent}} releases, a build identifier is appended to the semantic version of the base release in the format `+build{yyyymmddhhmm}`, where `{yyyymmddhhmm}` is the release timestamp of the build.

reference/fleet/toc.yml

@@ -19,6 +19,7 @@ toc:
       - file: fleet-server-monitoring.md
   - file: install-elastic-agents.md
     children:
+      - file: fleet-agent-release-process.md
       - file: install-fleet-managed-elastic-agent.md
       - file: install-standalone-elastic-agent.md
         children:

reference/fleet/upgrade-elastic-agent.md

@@ -80,7 +80,9 @@ With the right [subscription level](https://www.elastic.co/subscriptions), you c
 
 3. In the Upgrade agent window, select or specify an upgrade version and click **Upgrade agent**.
 
-    In certain cases the latest available {{agent}} version may not be recognized by {{kib}}. For instance, this occurs when the {{kib}} version is lower than the {{agent}} version. You can specify a custom version for {{agent}} to upgrade to by entering the version into the **Upgrade version** text field.
+    {{kib}} provides a list of the {{agent}} versions available for upgrade. A version number ending with a build identifier in the format `+build{yyyymmddhhmm}` indicates an [independent {{agent}} release version](./fleet-agent-release-process.md#independent-agent-releases).
+
+    In certain cases, the latest available {{agent}} version may not be recognized by {{kib}}. For instance, this occurs when the {{kib}} version is lower than the {{agent}} version. You can specify a custom version for {{agent}} to upgrade to by entering the version into the **Upgrade version** text field.
 
     :::{image} images/upgrade-agent-custom.png
     :alt: Menu for upgrading a single {{agent}}