add data model for the reference front-end (#31)

follows up from #31 to offer a data model focused on formalize to the point of incorporating any data attribute types to be stored in the `panel` db.

data model [requirements](https://git.fediversity.eu/Fediversity/meta/src/branch/main/architecture-docs/data-model-requirements.md):

- [x] specifying [entity relations](https://mermaid.js.org/syntax/entityRelationshipDiagram.html#relationship-syntax) e.g. many-to-many
- [x] migrating both deployed and staged configurations
- [x] deploying of applications using the same versions
- [x] retaining relevant application state
- handling of application-specific migration logic, such as to rewrite URLs as needed - note this requirement does not affect the model here, in the sense it is handled through code rather than through the database

closes Fediversity/Fediversity#103.

Reviewed-on: #31
Reviewed-by: Valentin Gagarin <valentin.gagarin@tweag.io>
Co-authored-by: cinereal <cinereal@riseup.net>
Co-committed-by: cinereal <cinereal@riseup.net>

architecture-docs/panel-data-model.mmd

@@ -0,0 +1,24 @@
+---
+title: Data model of sample web application
+---
+erDiagram
+    operator {
+        string username
+        string password_hash
+    }
+    deployment {
+        json deployed_configuration
+        option[string] staged_configuration
+        option[string] version
+    }
+    backup["back-up"] {
+        string bucket
+        string endpoint
+    }
+    keypair {
+        string access_key
+        string secret_key
+    }
+    operator ||--o{ deployment : has
+    deployment ||--o{ backup : has
+    backup ||--|{ keypair : authorises