Use generated diagrams in docs

Use images generated from Python diagrams in doc pages.

Signed-off-by: Davide Briani <[email protected]>

doc/pages/architecture/020-components.md

@@ -2,6 +2,8 @@
 
 Astarte is a distributed system interacting over AMQP, as explained in [Design Principles](010-design_principles.html). This is an overview of its main internal services.
 
+![Astarte architecture](assets/astarte-architecture.png)
+
 ## Pairing
 
 Pairing takes care of Device Authentication and Authorization. It interacts with Astarte's CA and orchestrates the way devices connect and interact with Transports. It also handles Device Registration. Agent, Device and Pairing interaction is described in detail [here](050-pairing_mechanism.html).

doc/pages/user/035-register_device.md

@@ -72,6 +72,8 @@ In this scenario, devices are imprinted with their *Credentials Secret* in the p
 Device might not even be connected to the Internet, whereas the machine running the Agent has access
 to the target Astarte Cluster and adequate Credentials for Registration.
 
+![Astarte Pairing registration](assets/astarte-pairing-registration.png)
+
 Once the Agent acquires the Device ID of the Device which should be registered, it issues the
 request to Astarte's Pairing API and obtains the Device's *Credentials Secret*. At this stage, the
 Agent is in charge of delivering the *Credentials Secret* to the Device the way it sees fit. As a

doc/pages/user/040-connect_device.md

@@ -22,6 +22,8 @@ Secret*. This *Credentials Secret* is the token the device uses for performing t
 routine, which results in the device obtaining its Credentials for accessing its designated
 Transport.
 
+![Astarte Pairing authentication](assets/astarte-pairing-authentication.png)
+
 A Device's *Credentials Secret* allows access to [Pairing API's Device REST
 API](https://docs.astarte-platform.org/1.0/api/?urls.primaryName=Pairing%20API#/device), which
 is then used for obtaining information about which Transports the Device can use for communicating,