Keep a Core Concepts area

We have still some quite unreleated topics which are in need of a new
home. Keep them accessible throught the navbar and links to it intact.

docs/core-concepts/behavior.md

@@ -4,6 +4,10 @@ sidebar_position: 4
 
 # API Behavior
 
+:::caution Deprecated
+This page will soon move into the [API reference](/api-reference) itself.
+:::
+
 This page documents the behavior of a `hydra-node` at the API layer. That is, how the system behaves given [ClientInputs](pathname:///haddock/hydra-node/Hydra-API-ClientInput.html#t:ClientInput) and what [ServerOutputs](pathname:///haddock/hydra-node/Hydra-API-ServerOutput.html#t:ServerOutput) are produced in response to it. See also the [API reference](/api-reference/) for more details about individual API messages. The only discrepancy is http `POST /commit` action which is not a state transition but a user action that submits a commit transaction which should produce `Committed` output.
 
 The formalism uses [UML statechart](https://en.wikipedia.org/wiki/UML_state_machine) language where transitions are labeled: `input [condition] / output`. When two outputs (e.g. `A` and `B`) are expected we write `A,B`, while `{A,B}` denotes mutual exclusiveness of outputs.

docs/core-concepts/index.md

@@ -0,0 +1,12 @@
+---
+sidebar_position: 1
+---
+
+# Core Concepts
+
+```mdx-code-block
+import DocCardList from '@theme/DocCardList';
+import {useDocsSidebar} from '@docusaurus/theme-common/internal';
+
+<DocCardList items={useDocsSidebar().items.filter(({ docId }) => docId != "index")}/>
+```

docs/docusaurus.config.js

@@ -165,6 +165,11 @@ const config = {
             label: "Developer Documentation",
             position: "left",
           },
+          {
+            to: "/core-concepts",
+            label: "Core Concepts",
+            position: "right",
+          },
           {
             to: "/topologies",
             label: "Topologies",