Add Mermaid schema

guides/assets/ravendb.md

+```mermaid
+erDiagram
+    converted_series {
+        BIGINT id PK "PRIMARY KEY"
+        VARCHAR sampling_point_id FK "FOREIGN KEY"
+        VARCHAR source FK "FOREIGN KEY"
+        VARCHAR target FK "FOREIGN KEY"
+        NUMERIC factor
+        VARCHAR createdby
+    }
+
+    sampling_points {
+        VARCHAR id PK "PRIMARY KEY"
+        VARCHAR media_monitored FK "FOREIGN KEY"
+        VARCHAR station_id FK "FOREIGN KEY"
+        VARCHAR measurement_regime FK "FOREIGN KEY"
+        BOOLEAN mobile
+        VARCHAR assessment_type FK "FOREIGN KEY"
+        VARCHAR station_classification FK "FOREIGN KEY"
+    }
+
+    stations {
+        VARCHAR id PK "PRIMARY KEY"
+        VARCHAR name
+        VARCHAR begin_position
+        VARCHAR end_position
+        VARCHAR network_id FK "FOREIGN KEY"
+    }
+
+    networks {
+        VARCHAR id PK "PRIMARY KEY"
+        VARCHAR name
+        VARCHAR media_monitored FK "FOREIGN KEY"
+        VARCHAR responsible_authority_id FK "FOREIGN KEY"
+    }
+
+    responsible_authorities {
+        VARCHAR id PK "PRIMARY KEY"
+        VARCHAR name
+        VARCHAR organisation
+    }
+
+    observations {
+        BIGSERIAL id PK "PRIMARY KEY"
+        VARCHAR sampling_point_id FK "FOREIGN KEY"
+        VARCHAR begin_position
+        VARCHAR end_position
+        NUMERIC value
+        TIMESTAMP from_time
+        TIMESTAMP to_time
+    }
+
+    USERGROUP {
+        INTEGER userid PK
+        INTEGER groupid PK
+    }
+    USER {
+        INTEGER id PK
+    }
+    GROUP {
+        INTEGER id PK
+    }
+
+    USER ||--o{ USERGROUP : "has"
+    GROUP ||--o{ USERGROUP : "includes"
+
+
+    eea_adjustmenttypes {
+        VARCHAR id PK "PRIMARY KEY"
+        VARCHAR label
+        VARCHAR uri
+    }
+
+    eea_areaclassifications {
+        VARCHAR id PK "PRIMARY KEY"
+        VARCHAR label
+        VARCHAR notation
+    }
+
+    eea_assessmentthresholdexceedances {
+        VARCHAR id PK "PRIMARY KEY"
+        VARCHAR label
+        VARCHAR uri
+    }
+
+    eea_assessmenttypes {
+        VARCHAR id PK "PRIMARY KEY"
+        VARCHAR label
+        VARCHAR notation
+    }
+
+    eea_concentrations {
+        VARCHAR id PK "PRIMARY KEY"
+        VARCHAR label
+        VARCHAR notation
+    }
+
+    eea_exceedancedescription {
+        INTEGER id PK "PRIMARY KEY"
+        VARCHAR name
+    }
+
+    eea_exceedancereason {
+        VARCHAR id PK "PRIMARY KEY"
+        VARCHAR label
+        VARCHAR uri
+    }
+
+    eea_exceedancetype {
+        NUMERIC id PK "PRIMARY KEY"
+        VARCHAR name
+    }
+
+    exceedancedescriptions {
+        VARCHAR id PK "PRIMARY KEY"
+        VARCHAR attainment_id FK "FOREIGN KEY"
+        BOOLEAN exceedances
+        INTEGER exceedance_type FK "FOREIGN KEY"
+        NUMERIC max_value
+        VARCHAR adjustment_type FK "FOREIGN KEY"
+        NUMERIC surface_area
+        INTEGER exposed_population
+        INTEGER population_reference_year
+        VARCHAR area_classification FK "FOREIGN KEY"
+        VARCHAR exceedance_reason FK "FOREIGN KEY"
+        INTEGER exceedancedescription_element FK "FOREIGN KEY"
+    }
+
+    converted_series ||--|{ sampling_points : "sampling_point_id"
+    sampling_points ||--|{ stations : "station_id"
+    stations ||--|{ networks : "network_id"
+    networks ||--|{ responsible_authorities : "responsible_authority_id"
+    observations ||--|{ sampling_points : "sampling_point_id"
+    users ||--|{ usergroup : "id"
+    group_table ||--|{ usergroup : "id"
+    exceedancedescriptions ||--|{ eea_exceedancetype : "exceedance_type"
+    exceedancedescriptions ||--|{ eea_adjustmenttypes : "adjustment_type"
+    exceedancedescriptions ||--|{ eea_areaclassifications : "area_classification"
+    exceedancedescriptions ||--|{ eea_exceedancereason : "exceedance_reason"
+    exceedancedescriptions ||--|{ eea_exceedancedescription : "exceedancedescription_element"
+```
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