docs: Add database concept and schema

doc/modules/ROOT/db-schema.adoc

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+# Items database object and table  schema
+
+The base schema of the item objects are data and link object, which have both a
+type. This way you could model the description "Chassis contains Mainboard" or
+"Service runs on Server".
+
+.Object schema concept
+[plantuml, format=svg]
+----
+@startuml
+
+left to right direction
+scale 200 width
+
+header Concept
+title General object and link
+
+object data1
+object data2
+object link1
+
+data1 : id: "uuid"
+data1 : type: "data type"
+data1 : data: "json"
+
+data2 : id: "uuid"
+data2 : type: "data type"
+data2 : data: "json"
+
+link1 : id: "uuid"
+link1 : type: "link type"
+link1 : from: "data.id"
+link1 : to: "data.id"
+
+
+data1::id <- link1::from
+link1::to -> data2::id
+@enduml
+----
+
+.Object schema example
+[plantuml, format=svg]
+----
+@startuml
+
+left to right direction
+scale 400 width
+
+header Example
+title Chassis contains Mainboard
+
+object data3
+object data4
+object link2
+
+data3 : id: "afd61751-dbee-4010-91dc-1263155d6a56"
+data3 : type: "Chassis"
+data3 : data: "{\n"serial": "12345678"\n}"
+
+data4 : id: "96dcf778-0875-437f-adc0-03545f9f06be"
+data4 : type: "Mainboard"
+data4 : data: "{\n "serial": "ABCDEFGHI"\n}"
+
+link2 : id: "f4402dbb-05db-4903-a378-5bff14cf91ca"
+link2 : type: "contains"
+link2 : from: "afd61751-dbee-4010-91dc-1263155d6a56"
+link2 : to: "96dcf778-0875-437f-adc0-03545f9f06be"
+
+
+data3::id <- link2::from
+link2::to -> data4::id
+
+@enduml
+
+----
+
+.Table schema
+[plantuml, format=svg]
+----
+@startuml
+
+left to right direction
+' scale 350 width
+' skinparam linetype ortho
+
+entity link_schema #eee8d5 {
+    * id: "uuid"
+    --
+    * type: "a|b|c"
+}
+
+entity link #fdf6e3 {
+    * id: "uuid"
+    --
+    * type: "schema_link"
+    * from: "data.id uuid"
+    * to: "data.id uuid"
+}
+
+note right of link_schema #add1b2
+    link_schema has only an 'object_type'
+end note
+
+entity data_schema #eee8d5 {
+    * id: "uuid"
+    --
+    * type: "x|y|z"
+    * schema: "json"
+}
+
+entity data #fdf6e3 {
+    * id: "uuid"
+    --
+    * type: "data_schema"
+    * data: "data_schema.schema json"
+}
+
+
+link::from -> data::id #A9DCDF
+data::id <- link::to #A9DCDF
+
+
+link_schema::type -> link::type
+
+data_schema::type -> data::type
+data::data <- data_schema::schema
+
+
+
+
+@enduml
+----