docs(mongodb): update documentation with images section and fixes

- Add full README content (deployment modes, notes sections)
- Add Image configuration section (pmm, backup images)
- Add configServerSize sharding parameter
- Fix S3 credential defaults (empty instead of placeholder)

Co-Authored-By: Claude <[email protected]>
Signed-off-by: Aleksei Sviridkin <[email protected]>

Author: lexfrei

content/en/docs/applications/mongodb.md

@@ -12,7 +12,41 @@ metadata: https://github.com/cozystack/website/blob/main/content/en/docs/applica
 source: https://github.com/cozystack/cozystack/blob/main/packages/apps/mongodb/README.md
 -->
 
-Managed MongoDB service
+
+MongoDB is a popular document-oriented NoSQL database known for its flexibility and scalability.
+The Managed MongoDB Service provides a self-healing replicated cluster managed by the Percona Operator for MongoDB.
+
+## Deployment Details
+
+This managed service is controlled by the Percona Operator for MongoDB, ensuring efficient management and seamless operation.
+
+- Docs: <https://docs.percona.com/percona-operator-for-mongodb/>
+- Github: <https://github.com/percona/percona-server-mongodb-operator>
+
+## Deployment Modes
+
+### Replica Set Mode (default)
+
+By default, MongoDB deploys as a replica set with the specified number of replicas.
+This mode is suitable for most use cases requiring high availability.
+
+### Sharded Cluster Mode
+
+Enable `sharding: true` for horizontal scaling across multiple shards.
+Each shard is a replica set, and mongos routers handle query routing.
+
+## Notes
+
+### External Access
+
+When `external: true` is enabled:
+- **Replica Set mode**: Traffic is load-balanced across all replica set members. This works well for read operations, but write operations require connecting to the primary. MongoDB drivers handle primary discovery automatically using the replica set connection string.
+- **Sharded mode**: Traffic is routed through mongos routers, which handle both reads and writes correctly.
+
+### Credentials
+
+On first install, the credentials secret will be empty until the Percona operator initializes the cluster.
+Run `helm upgrade` after MongoDB is ready to populate the credentials secret with the actual password.
 
 ## Parameters
 
@@ -31,13 +65,23 @@ Managed MongoDB service
 | `version`          | MongoDB major version to deploy.                                                                                                  | `string`   | `v8`    |
 
 
+### Image configuration
+
+| Name            | Description                            | Type     | Value                                  |
+| --------------- | -------------------------------------- | -------- | -------------------------------------- |
+| `images`        | Container images used by the operator. | `object` | `{}`                                   |
+| `images.pmm`    | PMM client image for monitoring.       | `string` | `percona/pmm-client:2.44.0`            |
+| `images.backup` | Percona Backup for MongoDB image.      | `string` | `percona/percona-backup-mongodb:2.8.0` |
+
+
 ### Sharding configuration
 
 | Name                                | Description                                                        | Type       | Value   |
 | ----------------------------------- | ------------------------------------------------------------------ | ---------- | ------- |
 | `sharding`                          | Enable sharded cluster mode. When disabled, deploys a replica set. | `bool`     | `false` |
 | `shardingConfig`                    | Configuration for sharded cluster mode.                            | `object`   | `{}`    |
 | `shardingConfig.configServers`      | Number of config server replicas.                                  | `int`      | `3`     |
+| `shardingConfig.configServerSize`   | PVC size for config servers.                                       | `quantity` | `3Gi`   |
 | `shardingConfig.mongos`             | Number of mongos router replicas.                                  | `int`      | `2`     |
 | `shardingConfig.shards`             | List of shard configurations.                                      | `[]object` | `[...]` |
 | `shardingConfig.shards[i].name`     | Shard name.                                                        | `string`   | `""`    |
@@ -67,8 +111,8 @@ Managed MongoDB service
 | `backup.retentionPolicy` | Retention policy (e.g. "30d").                         | `string` | `30d`                               |
 | `backup.destinationPath` | Destination path for backups (e.g. s3://bucket/path/). | `string` | `s3://bucket/path/to/folder/`       |
 | `backup.endpointURL`     | S3 endpoint URL for uploads.                           | `string` | `http://minio-gateway-service:9000` |
-| `backup.s3AccessKey`     | Access key for S3 authentication.                      | `string` | `<your-access-key>`                 |
-| `backup.s3SecretKey`     | Secret key for S3 authentication.                      | `string` | `<your-secret-key>`                 |
+| `backup.s3AccessKey`     | Access key for S3 authentication.                      | `string` | `""`                                |
+| `backup.s3SecretKey`     | Secret key for S3 authentication.                      | `string` | `""`                                |
 
 
 ### Bootstrap (recovery) parameters
@@ -79,3 +123,4 @@ Managed MongoDB service
 | `bootstrap.enabled`      | Whether to restore from a backup.                         | `bool`   | `false` |
 | `bootstrap.recoveryTime` | Timestamp for point-in-time recovery; empty means latest. | `string` | `""`    |
 | `bootstrap.backupName`   | Name of backup to restore from.                           | `string` | `""`    |
+