Fix all Doxygen errors and make any future errors fatal (#118) ...

* 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

Fix some build errors

Update UserAllocatedEvent.h

Update Event.h

Update equeue.h

Update EventQueue.h

Update equeue.h

extend the event generation int

bigger equeue generation size and only increment when needed ...

to mitigate against generation clash

Remove trailing space

Tweak events API update and update Greentea tests accordingly

Minor formatting tweak to fix file permissions

Further whitespace fixes

Fix astyle issues

Remove unecessary namespace qualified ...

(Also minor whitespace correction)

Fix some whitespace issues

Updated event period handling and greentea test ...

non_periodic constant has been moved out of the Event class and
made static within the events namespace so that it is available
both internally within the class and externally.

The MBED_ASSERT has been changed to MBED_WARNING.

The greentea test has been updated:
1) timings reduced to make the test cases run faster
2) The call handler simplified

Update events period method to check for invalid values ...

The period method currently allows any ms value positive, negative
and zero. A negative value means dispatch a non periodic event
ie just once. 0 is unspecified behaviour. This commit forces the
user to use either positive periods or a new constant non_periodic
and should any other value be provided will default to
non_periodic.

A Greentea test case is also provided to check this works as
expected.

Review feedback - capitilise argument descriptions

Update Event period and delay parameter comments

Fix the time units stated in the delay and period functon headers ...

delay() and period() still stated that the units were milliseconds
whereas in fact they are now a Chrono duration.

Add SPDX license identifier to Arm files ...

Add license identifier to files which Arm owns the copyright to,
and contain either BSD-3 or Apache-2.0 licenses. This is to address
license errors raised by scancode analysis.

Restructure events directory & move tests