* Start working on fixing doxygen warnings

* Upgrade doxyfile to Doxygen 1.9.1

* Clean up some unmatched groups and add some exclusions

* Fix docs for rtos::ThisThread not showing up

* Fix up KV store docs

* Fix incorrect @files, some more incorrect groups

* Fix lots of broken links

* Start fixing mbedtls groups

* Fix more mbedtls docs

* Fix remaining mbedtls files with missing or incorrect grouping

* Fix a huge amount of PSA docs errors

* Fix a few more mbedtls docs errors

* Fix __cplusplus not defined for docs, clean up call in callback typedef, fix some docs errors

* Fix some broken doc links in HAL headers

* Fix more PSA doc errors

* Fix some doxygen errors in networking code

* Fix lots of network related doc errors

* Try and fix some CI failures

* Use Doxygen Action instead of doxygen in Mbed container to get the right version

* Try removing docs check

* Add first part of doxygen action

* Add next part of doxygen action

* Fix yaml syntax error

* Add in license check fix

* Try using verbose scancode

* Fix some more docs issues, workaround scancode-toolkit#3179

* Clean up the LoRaRadio get_state() function on each implementation.  Also fix some other compile and doc errors.

* Fix all remaining doxygen errors

* Fix escaping, for realsies this time

* Remove Doxygen unsupported config options, fix some more get_status() declarations

* Fix some UNITTESTS build errors

* Fix one more compile error

* Another unittest error

Many test stub functions are meant to return a value, but weren't. Clang
would generate a warning for each instance where we weren't returning
anything in a function that was meant to return a value.

    warning: non-void function does not return a value [-Wreturn-type]

For a specific example, my_radio::time_on_air() is supposed to return a
uint32_t, but wasn't returning anything. We'll return a zero instead of
relying on undefined behavior.

Without this, clang 11.0.1 was generating a virtual function
implementation with a `ud2` instruction to abort at run-time, causing
some execution of some unit tests to abort.

    Running main() from gmock_main.cc
    [==========] Running 10 tests from 1 test suite.
    [----------] Global test environment set-up.
    [----------] 10 tests from Test_LoRaPHYUS915
    [ RUN      ] Test_LoRaPHYUS915.constructor
    [       OK ] Test_LoRaPHYUS915.constructor (0 ms)
    [ RUN      ] Test_LoRaPHYUS915.restore_default_channels
    [       OK ] Test_LoRaPHYUS915.restore_default_channels (0 ms)
    [ RUN      ] Test_LoRaPHYUS915.rx_config
    [       OK ] Test_LoRaPHYUS915.rx_config (0 ms)
    [ RUN      ] Test_LoRaPHYUS915.tx_config
    Process 35669 stopped
    * thread #1, name = 'lorawan-loraphy-', stop reason = signal SIGILL: privileged instruction
        frame #0: 0x0000000000276f73 lorawan-loraphy-us915-unittest`my_radio::time_on_air(this=0x0000000800c2b048, modem=MODEM_LORA, pkt_len='\0') at Test_LoRaPHYUS915.cpp:90:5
       87       };
       88
       89       virtual uint32_t time_on_air(radio_modems_t modem, uint8_t pkt_len)
    -> 90       {
       91       };
       92
       93       virtual bool perform_carrier_sense(radio_modems_t modem,
    (lldb) disassemble --pc
    lorawan-loraphy-us915-unittest`my_radio::time_on_air:
    ->  0x276f73 <+67>: ud2
        0x276f75:       int3
        0x276f76:       int3
        0x276f77:       int3
    (lldb)

```
connectivity
├── netsocket
├── lorawan
│   ├── mbed_lib.json // nanostack-interface's mbed_lib.json
│   ├── lorastack
│   ├── tests
├── nanostack
├── cellular
├── ...

```

This PR is a part of a wider \connectivity refactoring and implements the above new directory structure for \lorawan.