* 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

Mesh api: Added PHY mode, channel plan IDs and configuration functions

Updated based on review comments.

Clarify set_network_size usage and possible parameter values.
Clarify wisun-network-size help text in mbed-mesh-api/mbed_lib.json

Corrected Wi-SUN certificate add and remove functions to work properly
with .json certificates. .json certificates are used only if certificates
added by add functions are not set. Corrected certificate add and remove
functions to work properly and improved function descriptions.

-Added get, set and validate API for device min sens.
-Added get API for CCA threshold table.

-Adding new parameters for Wi-SUN interface information.

```
connectivity
├── netsocket
├── lwipstack
├── nanostack
│   ├── mbed_lib.json // nanostack-interface's mbed_lib.json
│   ├── include
│   │   └── nanostack // headers from features/netsocket/nanostack-interface
│   ├── source        // sources from features/netsocket/nanostack-interface
│   ├── coap-service  // used by Thread only - deprecate?
│   ├── mbed-mesh-api
│   ├── nanostack-hal-mbed-cmsis-rtos // HAL porting layer for Nanostack on mbed with CMSIS-RTOS
│   ├── sal-stack-nanostack
│   └── sal-stack-nanostack-eventloop
├── cellular                // previously in features/cellular/framework/
├── ...

```

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