add concept map docs

building/tracks/concept-map.md

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+# Concept Map
+
+The concept map is one of the main methods to illustrate the progression that a student takes through the concept exercises.
+
+## Design Goals
+
+- Provide a meaningful concept map of ideas from simple to complex ideas.
+- Provide a pathway towards fluency in a language.
+- Illustrate the progression through the course [track] contents.
+
+## Structure
+
+- Nodes
+  - Each concept is represented by a node on the concept map.
+  - Provide a quick reference with their style to indicate the degree of progression.
+    - Mastered: Concepts whose learning exercise and all associate practice exercise is complete.
+    - Complete: Concepts whose learning exercise is complete, but one or more practice exercise is not complete.
+    - Available: Concepts whose learning exercise has not been completed.
+    - Locked: Concepts whose learning exercise requires one or more pre-requisite exercise to be completed.
+- Paths
+  - Provide a relation of one concept to the next.
+  - Provide a relation indicating the building blocks of a concept back to the root.
+
+## Configuration
+
+The configuration of the concept map is determined by details of the `config.json` associated to a track.
+Specifically the specification of the concept exercises determines the shape and layout of the concept map.
+
+> You can view the code used to compute the concept map specification on GitHub: [Exercism/website: determine_concept_map_layout.rb][github-concept-code]
+
+A concept exercise specifies its prerequites and also the concept it teaches on completion.
+If we translate this to the terms of a [graph][wikipedia-graph]:
+- A concept represents a _vertex_ (also known as a _node_)
+- A concept exercise determines one or more _directed edges_ between two _vertices_ (_nodes_)
+
+It is important to note, not all edges that could be specified from the `config.json` appear in the result -- only the edges from the preceeding level are selected.
+
+```mermaid
+flowchart TD
+    Basics --> Integers
+    Integers --> Strings
+    Basics --Edge Removed--> Strings
+
+    linkStyle 2 stroke-width:2px,fill:none,stroke:red;
+```
+
+[github-concept-code]: https://github.com/exercism/website/blob/e47525f66b036662138e5e41055430fe7963a282/app/commands/track/determine_concept_map_layout.rb
+[wikipedia-graph]: https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)