esp_modem: Moved to component folder

components/esp_modem/docs/internal_design.md

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+# Internal design
+
+## Design decisions
+
+* Use C++ with additional C API
+
+* Use exceptions
+  - Use macro wrapper over `try-catch` blocks when exceptions off (use `abort()` if `THROW()`) 
+
+* Initializes and allocates in the constructor (might throw)
+  - easier code with exceptions ON, with exceptions OFF alloc/init failures are not treated as runtime error (program aborts)
+  - break down long initialization in constructor into more private methods
+
+* Implements different devices using inheritance from `GenericModule`, which is the most general implementation of a common modem
+  - Internally uses templates with device specialization (modeled as `DCE<SpecificModule>`) which could be used as well for some special cases, 
+  such as implantation of a minimal device (ModuleIf), add new AT commands (oOnly in compile time), or using the Module with DTE only (no DCE, no Netif) for sending AT commands without network
+  
+## 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.
+
+## CMUX terminal
+
+The below diagram depicts the idea of using CMUX terminal mode using the CMuxInstance 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 CMux class, which consumes the original terminal and uses its read/write methods
+to multiplex the terminal.
+
+![CMUX Terminal](CMux_collaboration.png)