Add a diagram for CAs and Certs usage between agent, fleet-server and ES (#5263)

* add a diagram for CAs and Certs usage

Author: AndersonQ

docs/agent-fleet-server-ES-CAs-certificates-usage.md

@@ -0,0 +1,146 @@
+# TLS between Elastic AGent <-> Fleet Server <-> Elasticsearch
+
+This shows the different certificates and certificate authorities used by the Elastic Agent, Fleet Server and Elasticsearch for the control plane communication and their respective cli flags when installing the Elastic Agent.
+
+ - Elastic Agent (client) makes HTTPS requests to Fleet Server (server)
+ - Fleet Server (client) makes HTTPS requests to Elasticsearch (server)
+
+## TLS
+
+For the TLS case, the following is used:
+ - fleet-ca: the certificate authority for the certificate presented by Fleet Server
+ - fleet-cert: the TLS certificate Fleet Server presents when a client makes an HTTPS request
+ - fleet-cert-key: the private key for Fleet Server's TLS certificate (not shown in the diagram)
+ - es-ca: the certificate authority for the certificate presented by Elasticsearch
+
+```shell
+elastic-agent install --url=https://your-fleet-server.elastic.co:443 \
+--certificate-authorities=/path/to/fleet-ca \
+--fleet-server-es=https://es.elastic.com:443 \
+--fleet-server-es-ca=/path/to/es-ca \
+--fleet-server-cert=/path/to/fleet-cert \
+--fleet-server-cert-key=/path/to/fleet-cert-key \
+--fleet-server-service-token=FLEET-SERVER-SERVICE-TOKEN \
+--fleet-server-policy=FLEET-SERVER-POLICY-ID \
+--fleet-server-port=8220
+```
+```mermaid
+flowchart LR
+    subgraph TLS
+        elastic-agent
+        fleet-server
+        elasticsearch
+    end
+
+
+    fleet-server --> |**fleet-server: presents fleet-cert**
+      --fleet-server-cert=fleet-cert| elastic-agent
+
+    elastic-agent --> |**agent validates fleet-ca:**
+      --certificate-authorities=/path/to/fleet-ca| fleet-server
+
+    fleet-server --> | **fleet-server es-ca validates es-cert:**
+      --fleet-server-es-ca=es-ca| elasticsearch
+
+        elasticsearch --> | **elasticsearch presents es-cert**| fleet-server
+
+
+    subgraph elasticsearch
+        es-cert
+    end
+    subgraph fleet-server
+        fleet-cert
+        es-ca
+    end
+    subgraph elastic-agent
+        fleet-ca
+    end
+```
+
+## mTLS
+
+For the mTLS case, the following is used:
+- agent-ca: the certificate authority for the certificate presented by the Elastic Agent
+- agent-cert: the client TLS certificate Elastic Agent presents to Fleet Server
+- agent-cert-key: the private key for the Elastic Agent's TLS certificate (not shown in the diagram)
+
+- fleet-ca: the certificate authority for the certificate presented by Fleet Server
+- fleet-cert: the TLS certificate Fleet Server presents when a client makes an HTTPS request
+- fleet-cert-key: the private key for Fleet Server's TLS certificate (not shown in the diagram)
+
+- fleet-es-ca: the certificate authority for the client TLS certificate presented by Fleet Server to Elasticsearch
+- fleet-es-cert: the client TLS certificate Fleet Server presents to Elasticsearch
+- fleet-es-cert-key: the private key for Fleet Server's client TLS certificate (not shown in the diagram)
+
+- es-ca: the certificate authority for the certificate presented by Elasticsearch
+- es-cert: the TLS certificate Elasticsearch presents when a client makes an HTTPS request
+
+```shell
+elastic-agent install --url=https://your-fleet-server.elastic.co:443 \
+--certificate-authorities=/path/to/fleet-ca,/path/to/agent-ca \
+--elastic-agent-cert=/path/to/agent-cert \
+--elastic-agent-cert-key=/path/to/agent-cert-key \
+--fleet-server-es=https://es.elastic.com:443 \
+--fleet-server-es-ca=/path/to/es-ca \
+--fleet-server-es-cert=/path/to/fleet-es-cert \
+--fleet-server-es-cert-key=/path/to/fleet-es-cert-key \
+--fleet-server-cert=/path/to/fleet-cert \
+--fleet-server-cert-key=/path/to/fleet-cert-key \
+--fleet-server-client-auth=required \
+--fleet-server-service-token=FLEET-SERVER-SERVICE-TOKEN \
+--fleet-server-policy=FLEET-SERVER-POLICY-ID \
+--fleet-server-port=8220
+```
+```mermaid
+flowchart LR
+    subgraph mTLS
+        elastic-agent((elastic-agent))
+
+        fleet-server((fleet-server))
+        elasticsearch((elasticsearch))
+
+    end
+
+
+    elastic-agent --> |**agent: fleet-ca validates fleet-cert:**
+      --certificate-authorities=/path/to/fleet-ca| fleet-server
+
+    elastic-agent --> |**agent presents agent-cert:**
+      --elastic-agent-cert=agent-cert| fleet-server
+
+
+    fleet-server --> |**fleet-server: presents fleet-cert**
+      --fleet-server-cert=fleet-cert| elastic-agent
+
+    fleet-server --> |**fleet-server: agent-ca validates agent-cert**
+      --certificate-authorities=fleet-ca,agent-ca| elastic-agent
+
+
+    fleet-server --> | **fleet-server es-ca validates es-cert:**
+      --fleet-server-es-ca=es-ca| elasticsearch
+
+    fleet-server --> | **fleet-server presents fleet-es-cert:**
+      --fleet-server-es-cert=fleet-es-cert| elasticsearch
+
+
+    elasticsearch --> | **elasticsearch presents es-cert**| fleet-server
+    elasticsearch --> | **elasticsearch: fleet-es-ca validates fleet-es-cert**| fleet-server
+
+
+    subgraph elastic-agent
+        fleet-ca
+        agent-cert
+    end
+    subgraph fleet-server
+        fleet-cert
+        fleet-es-cert
+        agent-ca
+        es-ca
+    end
+    subgraph elasticsearch
+        es-cert
+        fleet-es-ca
+    end
+```
+
+