Add documentation about BLE connection and services.

doc/ble/ans_sequence.puml

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+@startuml
+group Get configuration from server
+Pinetime -> CompanionApp : Read request (Supported 'new' notification categories - 0x2A47)
+CompanionApp --> Pinetime: Response (0xff = all categories)
+
+Pinetime -> CompanionApp : Read request (Supported 'unread' notification categories - 0x2A48)
+CompanionApp --> Pinetime: Response (0xff = all categories)
+end
+
+group Configure the server
+Pinetime -> CompanionApp : Configure 'new' notifications categories : Accept all (Write [0, 0xFF] to Control Point - 0x2A44)
+activate CompanionApp
+deactivate CompanionApp
+Pinetime -> CompanionApp : Configure 'unread' notifications categories : Accept all (Write [1, 0xFF] to Control Point - 0x2A44)
+activate CompanionApp
+deactivate CompanionApp
+end
+
+group Server sends a notification
+CompanionApp -> Pinetime : Notify new_alert (0x2A46)
+activate Pinetime
+note over Pinetime
+    Pinetime FW handles the
+    notification (display on UI)
+end note
+deactivate Pinetime
+end
+
+@enduml

doc/ble/connection_sequence.puml

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+@startuml
+Pinetime --> CompanionApp: Start advertising
+
+group BLE Connection
+CompanionApp -> Pinetime: Connection request
+CompanionApp <-> Pinetime: Connection parameters negociation, security procedure,...
+end
+
+group Service Discovery
+CompanionApp <-> Pinetime: Discover service 0x1805 (CTS)
+CompanionApp <-> Pinetime: Discover service 0x1811 (ANS)
+end
+
+@enduml