Rearrangement of the conceptual documents. (ros2#3732)

* Rearrangement of the conceptual documents.

Lay it out in a way so we can incrementally explain
the necessary concepts as readers need.

Signed-off-by: Chris Lalancette <[email protected]>
Co-authored-by: Michael Carroll <[email protected]>

Author: clalancette

source/Concepts.rst

@@ -1,5 +1,3 @@
-.. _ConceptsHome:
-
 Concepts
 ========
 
@@ -8,113 +6,9 @@ Conceptual overviews provide relatively high-level, general background informati
 .. toctree::
    :maxdepth: 1
 
-   Concepts/About-Domain-ID
-   Concepts/About-Different-Middleware-Vendors
-   Concepts/About-Logging
-   Concepts/About-Quality-of-Service-Settings
-   Concepts/About-ROS-2-Client-Libraries
-   Concepts/About-ROS-Interfaces
-   Concepts/About-ROS-2-Parameters
-   Concepts/About-Executors
-   Concepts/About-Topic-Statistics
-   Concepts/About-Command-Line-Tools
-   Concepts/About-RQt
-   Concepts/About-Composition
-   Concepts/About-Catment
-   Concepts/About-Cross-Compilation
-   Concepts/About-Security
-   Concepts/About-Tf2
-
-
-The Core Stack Developer Concepts are much more detailed conceptual articles intended for developers who plan modify or contribute to the ROS 2 core:
-
-.. toctree::
-  :maxdepth: 2
-
-  Concepts/About-Build-System
-  Concepts/About-Internal-Interfaces
-  Concepts/About-Middleware-Implementations
-  Concepts/About-ROS-2-Client-Libraries
-
-Quick overview of ROS 2 Concepts
---------------------------------
-
-.. contents::
-   :local:
-
-ROS 2 is a middleware based on an anonymous publish/subscribe mechanism that allows for message passing between different ROS processes.
-
-At the heart of any ROS 2 system is the ROS graph.
-The ROS graph refers to the network of nodes in a ROS system and the connections between them by which they communicate.
-
-Graph Concepts
-^^^^^^^^^^^^^^
-
-
-* Nodes: A node is an entity that uses ROS to communicate with other nodes.
-* Messages: ROS data type used when subscribing or publishing to a topic.
-* Topics: Nodes can publish messages to a topic as well as subscribe to a topic to receive messages.
-* Discovery: The automatic process through which nodes determine how to talk to each other.
-
-Nodes
-^^^^^
-
-A node is a participant in the ROS graph.
-ROS nodes use a ROS client library to communicate with other nodes.
-Nodes can publish or subscribe to Topics.
-Nodes can also provide or use Services and Actions.
-There are configurable Parameters associated with a node.
-Connections between nodes are established through a distributed discovery process.
-Nodes may be located in the same process, in different processes, or on different machines.
-These concepts will be described in more detail in the sections that follow.
-
-Client Libraries
-^^^^^^^^^^^^^^^^
-
-ROS client libraries allow nodes written in different programming languages to communicate.
-There is a core ROS client library (RCL) that implements common functionality needed for the ROS APIs of different languages.
-This makes it so that language-specific client libraries are easier to write and that they have more consistent behavior.
-
-The following client libraries are maintained by the ROS 2 team:
-
-
-* rclcpp = C++ client library
-* rclpy = Python client library
-
-Additionally, other client libraries have been developed by the ROS community.
-See the :doc:`ROS 2 Client Libraries <Concepts/About-ROS-2-Client-Libraries>` article for more details.
-
-Discovery
-^^^^^^^^^
-
-Discovery of nodes happens automatically through the underlying middleware of ROS 2.
-It can be summarized as follows:
-
-#. When a node is started, it advertises its presence to other nodes on the network with the same ROS domain (set with the ROS_DOMAIN_ID environment variable).
-   Nodes respond to this advertisement with information about themselves so that the appropriate connections can be made and the nodes can communicate.
-#. Nodes periodically advertise their presence so that connections can be made with new-found entities, even after the initial discovery period.
-#. Nodes advertise to other nodes when they go offline.
-
-Nodes will only establish connections with other nodes if they have compatible :doc:`Quality of Service <../Tutorials/Demos/Quality-of-Service>` settings.
-
-Take the :ref:`talker-listener demo <talker-listener>` for example.
-Running the C++ talker node in one terminal will publish messages on a topic,
-and the Python listener node running in another terminal  will subscribe to messages on the same topic.
-
-You should see that these nodes discover each other automatically, and begin to exchange messages.
-
-Security
-^^^^^^^^
-
-ROS 2 includes the ability to secure communications among nodes within the ROS 2 computational graph.
-Similar to discovery, security happens through the underlying ROS 2 middleware (provided it has support for the corresponding security plugins).
-No additional software installation is needed to enable security; however, the middleware requires configuration files for each ROS graph participant.
-These files enable encryption and authentication, and define policies both for individual nodes and for the overall ROS graph.
-ROS 2 also adds a master "on/off" switch to control security behavior.
-
-ROS utilities can create the authoritative `trust anchor <https://en.wikipedia.org/wiki/Trust_anchor>`_ for a ROS application, or an external certificate authority can be used.
-
-See the :doc:`ROS 2 Security <Concepts/About-Security>` article for additional details or ROS security features.
+   Concepts/Basic
+   Concepts/Intermediate
+   Concepts/Advanced
 
 
 Related Content
@@ -126,4 +20,3 @@ For a brief video introduction to ROS 2, see this community contributed content:
 
 * `Getting started with ROS Part 1: Nodes, Parameters and Topics <https://youtu.be/46TPAKXBOF8>`_
 * `Getting started with ROS Part 2: Services and Actions <https://youtu.be/keZAJ83eEoM>`_
-