Add e2e sequence diagrams

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

docs/internal/diagrams.md

+# Flux Diagrams
+
+## Cluster sync from Git
+
+```mermaid
+sequenceDiagram
+    actor me
+    participant git as Git<br><br>repository
+    participant sc as Flux<br><br>source-controller
+    participant kc as Flux<br><br>kustomize-controller
+    participant kube as Kubernetes<br><br>api-server
+    participant nc as Flux<br><br>notification-controller
+    me->>git: 1. git push
+    sc->>git: 2. git pull
+    sc->>sc: 3. build artifact for revision
+    sc->>kube: 4. update status for revision
+    sc-->>nc: 5. emit events
+    kube->>kc: 6. notify about new revision
+    kc->>sc: 7. fetch artifact for revision
+    kc->>kc: 8. build manifests to objects
+    kc-->>kc: 9. decrypt secrets
+    kc->>kube: 10. validate objects
+    kc->>kube: 11. apply objects
+    kc-->>kube: 12. delete objects
+    kc-->>kube: 13. wait for readiness
+    kc->>kube: 14. update status for revision
+    kc-->>nc: 15. emit events
+    nc-->>me: 16. send alerts for revision
+```
+
+## Helm release upgrade from Git
+
+```mermaid
+sequenceDiagram
+    actor me
+    participant git as Git<br><br>repository
+    participant sc as Flux<br><br>source-controller
+    participant hc as Flux<br><br>helm-controller
+    participant kube as Kubernetes<br><br>api-server
+    participant nc as Flux<br><br>notification-controller
+    me->>git: 1. git push
+    sc->>git: 2. git pull
+    sc->>sc: 3. build chart for revision
+    sc->>kube: 4. update chart status
+    sc-->>nc: 5. emit events
+    kube->>hc: 6. notify about new revision
+    hc->>sc: 7. fetch chart
+    hc->>kube: 8. get values
+    hc->>hc: 9. render and customize manifests
+    hc-->>kube: 10. apply CRDs
+    hc->>kube: 11. upgrade release
+    hc-->>kube: 12. run tests
+    hc-->>kube: 13. wait for readiness
+    hc->>kube: 14. update status
+    hc-->>nc: 15. emit events
+    nc-->>me: 16. send alerts
+```
+
+## Image update to Git
+
+```mermaid
+sequenceDiagram
+    actor me
+    participant oci as Image<br><br>repository
+    participant irc as Flux<br><br>image-reflector-controller
+    participant iac as Flux<br><br>image-automation-controller
+    participant kube as Kubernetes<br><br>api-server
+    participant nc as Flux<br><br>notification-controller
+    participant git as Git<br><br>repository
+    me->>oci: 1. docker push
+    irc->>oci: 2. list tags
+    irc->>irc: 3. match tags to policies
+    irc->>kube: 4. update status
+    irc-->>nc: 5. emit events
+    kube->>iac: 6. notify about new tags
+    iac->>git: 7. git clone
+    iac->>iac: 8. patch manifests with new tags
+    iac->>git: 9. git push
+    iac->>kube: 10. update status
+    iac-->>nc: 11. emit events
+    nc-->>me: 12. send alerts
+```
+