doc(diagrams): add some diagrams in mermaid format

doc/diagrams/ERDiagram.mmd

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+erDiagram
+journey||--|{routesegments: "consists of"
+routesegments||--||segment: "sorts"
+segment||--|{departure: "consists of"
+departure}|--|{station: "originates from"
+departure}|--||vehicle: uses
+vehicle||..|{line: "shared data"
+segment}|--||vehicle: uses
+stationlines}|..||station: "general connections"
+stationlines}|..||line: groups
+
+
+journey {
+  int id PK
+  string startstation
+  string endstation
+}
+routesegments {
+  int id PK
+  int journey FK
+  int segment FK
+  int segmentorder
+}
+segment {
+  int id PK
+  string ref
+  int startid FK
+  int endid FK
+  string vehicleid FK
+}
+departure {
+  int id PK
+  string ref
+  string direction
+  string vehicle FK
+  string vehiclename
+  string station FK
+  string stationname
+  string arrivaltime
+  string departuretime
+}
+station {
+  string id PK
+  string name
+}
+stationlines {
+  int id PK
+  string stationid FK
+  string lineid FK
+}
+vehicle {
+  string vehicleid PK
+  string name
+  string line FK
+}
+line {
+  string lineid PK
+  string name
+  string mode
+  string destination
+  string direction
+  string fgcolor
+  string bgcolor
+}

doc/diagrams/flowchart_to_journey.mmd

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+flowchart TD
+Start-->stat[Search Station]-->selstat[Select station]
+selstat-->seldep["`Select departure:
+vehicle, direction & time`"]
+seldep-->selarr["`Select arrival station:
+when & where to exit`"] --> sw{switch?} --yes-->seldep
+sw--"no"-->fin([journey details])
+

doc/diagrams/sequencediagram.mmd

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+sequenceDiagram
+  participant api@{ "type": "entity"} as public transport API
+  participant ah as api_handler
+  participant ui as UI
+  participant th as transport_helper
+  participant db@{ "type":"database"} as database
+
+ui-->>ah: find Station
+ah-->>api: GET request w/ stationname
+api-->>ah: return json
+ah-->>th: provide json Map
+th-->>db: store object
+db-->>th: return id
+th-->>ah: return object
+ah-->>ui: display data
+
+
+
+