DOC: formulate pure SymPy amplitude model

.github/workflows/ci-docs.yml

@@ -1,4 +1,8 @@
 name: Documentation
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: |-
+    ${{ github.ref != format('refs/heads/{0}', github.event.repository.default_branch) }}
 on:
   pull_request:
     branches: [main]

docs/index.md

@@ -16,6 +16,7 @@ address to the issue ✅ [ComPWA/gluex-nstar#1](https://github.com/ComPWA/g
 
 ```{toctree}
 reaction
+symbolics
 pgamma-state
 branching-fraction
 ampform/etapi0p