Added internal desing document

esp_modem/docs/internal_desing.md

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+# Internal design
+
+## Design decisions
+
+* Use C++ (lambdas) wit C API
+* Use exceptions (implement nothrow? possibly using `assert()`)
+* Initializes and allocates in the constructor (might throw) 
+* Implements different devices with template specialization
+* Specific device abstraction overrides methods, but does not use virtual function dispatch (modeled as `DCE<SpecificModule>`)
+
+## DCE collaboration model
+
+The diagram describes how the DCE class collaborates with DTE, PPP and the device abstraction
+
+![DCE_architecture](DCE_DTE_collaboration.png)
+
+## Terminal inheritance
+
+Terminal is a class which can read or write data, and can handle callbacks when data are available. UART specialization
+is provided implementing these method using the uart driver.
+
+![TerminalInheritance](Terminal_inheritance.png)
+
+## CMUX terminal
+
+The below diagram depicts the idea of using CMUX terminal mode using the CMUXedTerminal class which is a terminal
+(it implements the basic read/write methods) interfacing arbitrary number of virtual terminals,
+but at the same time it is also composed of another terminal, the real terminal, which is multiplexed.
+
+![CMUX Terminal](CMux_collaboration.png)