Update Calliope terminology description in docs (#706)

* Add system description image

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ### User-facing changes
 
+|changed| "An overview of the Calliope terminology" information admonition to remove self-references and improve understandability.
+Now also includes a visual depiction of how the different defined components connect together (#699).
+
 |fixed| Area-based parameters have appropriate documented units of `area` rather than `area^2` (#701).
 
 |fixed| Technology capacity lower bound constraints so that `[cap-type]_min` (e.g., `flow_cap_min`) is not always enforced if the `purchased_units` variable is active (#643).

docs/advanced/solver.md

@@ -2,7 +2,7 @@
 
 ## Gurobi
 
-Refer to the [Gurobi manual](https://www.gurobi.com/documentation/), which contains a list of parameters.
+Refer to the [Gurobi manual](https://docs.gurobi.com/projects/optimizer/en/current/reference/parameters.html), which contains a list of parameters.
 Simply use the names given in the documentation (e.g. "NumericFocus" to set the numerical focus value).
 For example:
 

docs/creating/index.md

@@ -1,25 +1,22 @@
 # Creating a model
 
-In short, a Calliope model works like this: **supply technologies** can take a **source** from outside of the modeled system and turn it into a specific **carrier** in the system.
-The model specifies one or more **nodes** along with the technologies allowed at those nodes.
-**Transmission technologies** can move the same carrier from one node to another, while **conversion technologies** can convert one carrier into another at the same node.
-**Demand technologies** remove carriers from the system through a **sink**, while **storage technologies** can store carriers at a specific node.
-Putting all of these possibilities together allows a modeller to specify as simple or as complex a model as necessary to answer a given research question.
-
-??? info "An overview of the Calliope terminology"
-    The terminology defined here is used throughout the documentation and the model code and configuration files:
-
-    * **Technology**: a technology that produces, consumes, converts or transports carriers.
-    * **Node**: a site which can contain multiple technologies and which may contain other nodes for carrier balancing purposes.
-    * **Source**: a source of commodity that can (or must) be used by a technology to introduce carriers into the system.
-    * **Sink**: a commodity sink that can (or must) be used by a technology to remove carriers from the system.
-    * **Carrier**: a carrier that groups technologies together into the same network, for example `electricity` or `heat`.
-
-    As more generally in constrained optimisation, the following terms are also used:
-
-    * Parameter: a fixed coefficient that enters into model equations.
-    * Variable: a variable coefficient (decision variable) that enters into model equations.
-    * Constraint: an equality or inequality expression that constrains one or several variables.
+A Calliope model is a collection of interconnected technologies, nodes and carriers describing a real world system of flows.
+Usually, we consider those to be _energy_ flows, and most of what you will read in this documentation concerns energy systems.
+However, it is just as applicable to other types of flows, such as water!
+
+**Carriers** are commodities whose flows we track, e.g., electricity, heat, hydrogen, water, CO<sub>2</sub>.
+
+**Technologies** supply, consume, convert, store or transmit _carriers_, e.g., transmission lines/pipes, batteries, power plants, wind turbines, or home appliances.
+
+**Nodes** contain groups of _technologies_ and are usually geographic, e.g., a country, municipality or a single house.
+
+Flows can enter the system from **sources**, e.g., energy from the sun to power a solar panel, and can exit it into **sinks**, e.g., electricity consumed by household appliances.
+Unlike _carriers_, we do not explicitly track the type of commodity described by sources and sinks.
+
+Putting all of these possibilities together allows a modeller to create a model that is as simple or complex as necessary to answer a given research question.
+Calliope's syntax ensures these models are intuitive, and easy to understand and share.
+
+![Visual description of the Calliope terminology.](../img/description_of_system.svg)
 
 !!! example
     Refer to the [examples and tutorials section](../examples/index.md) for a more practical look at how to build a Calliope model.