docs: improve mTLS documentation structure and visibility

Reorganize Certificate secret reference section to prioritize mutual TLS
authentication discovery and reduce user friction. The previous structure
buried mTLS information within generic certificate documentation, causing
users to miss this important security feature.

Create dedicated sections for mTLS and CA-only authentication with complete
examples and clear explanations of field requirements. Use stringData format
in examples for better readability and align documentation structure with
other FluxCD components.

Signed-off-by: cappyzawa <cappyzawa@gmail.com>

docs/spec/v1beta2/imagerepositories.md

@@ -188,53 +188,98 @@ see the integration [docs](/flux/integrations/).
 ### Certificate secret reference
 
 `.spec.certSecretRef.name` is an optional field to specify a secret containing
-TLS certificate data. The secret can contain the following keys:
+TLS certificate data for secure communication. The secret must be of type 
+`Opaque` or `kubernetes.io/tls`.
 
-* `tls.crt` and `tls.key`, to specify the client certificate and private key used
-for TLS client authentication. These must be used in conjunction, i.e.
-specifying one without the other will lead to an error.
-* `ca.crt`, to specify the CA certificate used to verify the server, which is
-required if the server is using a self-signed certificate.
+#### Supported configurations
 
-If the server is using a self-signed certificate and has TLS client
-authentication enabled, all three values are required.
+- **Mutual TLS (mTLS)**: Client certificate authentication (provide `tls.crt` + `tls.key`, optionally with `ca.crt`)
+- **CA-only**: Server authentication (provide `ca.crt` only)
 
-The Secret should be of type `Opaque` or `kubernetes.io/tls`. All the files in
-the Secret are expected to be [PEM-encoded][pem-encoding]. Assuming you have
-three files; `client.key`, `client.crt` and `ca.crt` for the client private key,
-client certificate and the CA certificate respectively, you can generate the
-required Secret using the `flux create secret tls` command:
+#### Mutual TLS Authentication
+
+Mutual TLS authentication allows for secure client-server communication using
+client certificates stored in Kubernetes secrets. Both `tls.crt` and `tls.key`
+must be specified together for client certificate authentication. The `ca.crt`
+field is optional but required when connecting to servers with self-signed certificates.
+
+All the files in the Secret are expected to be [PEM-encoded][pem-encoding]. 
+Assuming you have three files; `client.key`, `client.crt` and `ca.crt` for the 
+client private key, client certificate and the CA certificate respectively, you 
+can generate the required Secret using the `flux create secret tls` command:
 
 ```sh
 flux create secret tls --tls-key-file=client.key --tls-crt-file=client.crt --ca-crt-file=ca.crt
 ```
 
-Example usage:
+##### Example: mTLS Configuration
 
 ```yaml
 ---
 apiVersion: image.toolkit.fluxcd.io/v1beta2
 kind: ImageRepository
 metadata:
-  name: example
+  name: example-mtls
   namespace: default
 spec:
   interval: 5m0s
   image: example.com
   certSecretRef:
-    name: example-tls
+    name: example-mtls-certs
 ---
 apiVersion: v1
 kind: Secret
 metadata:
-  name: example-tls
+  name: example-mtls-certs
   namespace: default
 type: kubernetes.io/tls # or Opaque
-data:
-  tls.crt: <BASE64>
-  tls.key: <BASE64>
-  # NOTE: Can be supplied without the above values
-  ca.crt: <BASE64>
+stringData:
+  tls.crt: |
+    -----BEGIN CERTIFICATE-----
+    <client certificate>
+    -----END CERTIFICATE-----
+  tls.key: |
+    -----BEGIN PRIVATE KEY-----
+    <client private key>
+    -----END PRIVATE KEY-----
+  ca.crt: |
+    -----BEGIN CERTIFICATE-----
+    <certificate authority certificate>
+    -----END CERTIFICATE-----
+```
+
+#### CA Certificate Authentication
+
+CA certificate authentication provides server authentication when connecting to
+container registries with self-signed or custom CA certificates. Only the `ca.crt`
+field is required for this configuration.
+
+##### Example: CA Certificate Configuration
+
+```yaml
+---
+apiVersion: image.toolkit.fluxcd.io/v1beta2
+kind: ImageRepository
+metadata:
+  name: example-ca
+  namespace: default
+spec:
+  interval: 5m0s
+  image: example.com
+  certSecretRef:
+    name: example-ca-cert
+---
+apiVersion: v1
+kind: Secret
+metadata:
+  name: example-ca-cert
+  namespace: default
+type: kubernetes.io/tls # or Opaque
+stringData:
+  ca.crt: |
+    -----BEGIN CERTIFICATE-----
+    <certificate authority certificate>
+    -----END CERTIFICATE-----
 ```
 
 **Warning:** Support for the `caFile`, `certFile` and `keyFile` keys have been