Prepare concept documentation page for the Update Manager (#240)

[#241] Prepare concept documentation page for the Update Manager
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Signed-off-by: Filip Fidanov [email protected]

web/site/content/docs/concepts/update-manager.md

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+---
+title: "Update manager"
+type: docs
+description: >
+  Empower the edge device for OTA updates.
+weight: 2
+---
+
+Update manager enables a lightweight core component which is capable to easily perform complex OTA update scenarios on a target device. The following capabilities are provided:
+
+* **Lightweight** - consists of a single core component which orchestrates the update process
+* **Flexible deployment** - supports different deployment models - natively, as an executable or container
+* **Unified API** - all domain agents utilize a unified Update Agent API for interacting with the Update Manager
+* **MQTT Connectivity** - the connectivity and communication between the Update Manager and domain agent is MQTT-based
+* **Multi-domain integration** - easily integrates, scales and performs complex update operations across multiple domains
+* **Default update agents** - integrates with the Kanto provided out-of-the box domain update agent implementation for deployment of container into the Kanto container management
+* **Pluggable architecture** - provides an extensible model for plug-in custom orchestration logic
+* **Asynchronous updates** - asynchronous and independent update process across the different domains
+* **Multi-staged updates** - the update process is organized into different stages
+* **Configurable** - offers a variety of configuration options for connectivity, supported domains, message reporting and etc
+
+![Update manager](/kanto/images/docs/concepts/update-manager.png)
+
+### How it works
+
+The update process is initiated by sending the desired state specification as an MQTT message towards the device, which is handled by the Update Manager component.
+
+The desired state specification in the scope of the Update Manager is a JSON-based document, which consists of multiple component definitions per domain, representing the desired state to be applied on the target device.
+A component in the same context means a single, atomic and updatable unit, for example, OCI-compliant container, software application or firmware image.
+
+Each domain agent is a separate and independent software component, which implements the Update Agent API for interaction with the Update Manager and manages the update logic for concrete domain. For example - container management.
+
+The Update Manager, operating as a coordinator, is responsible for processing the desired state specification, distributing the split specification across the different domain agents, orchestrating the update process via MQTT-based commands, collecting and consolidating the feedback responses from the domain update agents, and reporting the final result of the update campaign to the backend.
+
+As extra features and capabilities, the Update Manager enables reboot of the host after the update process is completed and reporting of the current state of the system to the backend.