Doc to explain how to solve pgp key downloard issue in air-gapped env (…

…#3375)

* Doc to explain how to solve pgp key downloard issue in air-gapped env

* Update pgp-workaround.md

* Adding starting version

(cherry picked from commit 10d9ba2)

docs/pgp-workaround.md

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+# PGP fallback URL in air-gapped environment workaround
+
+## Reminders
+
+Starting from version 8.9.0, when Elastic Agent tries to perform an upgrade, it first verifies the binary signature with the key bundled in the Agent.
+This process has a backup mechanism that will use the key coming from https://artifacts.elastic.co/GPG-KEY-elastic-agent instead of the one it already has.
+
+In an air-gapped environment, the Agent won't be able to download the remote key and therefore cannot be upgraded.
+
+## Workaround
+
+To resolve this issue, we need the Agent to download the remote key from a server accessible from the air-gapped environment.
+As this URL is not customizable, we will have to "trick" the system by pointing https://artifacts.elastic.co/ to another host that will have the file.
+
+## Examples
+
+All those examples will require a server in your air-gapped environment that will expose the key you will have downloaded from https://artifacts.elastic.co/GPG-KEY-elastic-agent.
+
+### Manual
+
+Edit the Agent's server hosts file to add the following content:
+```bash
+<YOUR_HOST_IP> artifacts.elastic.co
+```
+
+Linux hosts file path:
+```bash
+/etc/hosts
+```
+
+Windows hosts file path:
+```bash
+C:\Windows\System32\drivers\etc\hosts
+```
+
+### Puppet 
+
+```yaml
+host { 'elastic-artifacts':
+  ensure       => 'present'
+  comment      => 'Workaround for PGP check'
+  ip           => '<YOUR_HOST_IP>'
+}
+```
+
+### Ansible 
+
+```yaml
+- name  : 'elastic-artifacts'
+  hosts : 'all'
+  become: 'yes'  
+
+  tasks:
+    - name: 'Add entry to /etc/hosts'
+      lineinfile:
+        path: '/etc/hosts'
+        line: '<YOUR_HOST_IP> artifacts.elastic.co'
+```