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

Lots of usability improvements for the I2C API. Better docs and new top-level functions. (#64) ...

* Lots of usability improvements for the I2C API.  Better docs and new top-level functions.

* Document frequencies

* Tabs to spaces

* More style fixes

* Run astyle

* Clean up docs

* Add note about addressing, change 10 bit to 11 bit

* Fix spellcheck

* Fix paste error

* Oops, fix accidental change

Fix ABI differences between mbed-baremetal and mbed-os, remove PlatformMutex

rtos: Thread: Make stack allocation failure runtime catchable ...

When a Thread object's stack memory is not provided, its `start()`
member function dynamically allocates its stack from the heap. If
allocation fails, there is no way to catch it because
* `std::nothrow` is missing after the `new` keyword. As Mbed OS
is built with `-fno-exceptions` (C++ exceptions disabled), failed
allocation results in an unrecoverable fault.
* The attempted `nullptr` check, which doesn't work anyway due to
the first point, is an assertion instead of error returning.
Assertions should be used as a development tool to ensure code
behaves correctly. But out-of-memory is a completely valid runtime
situation.

This commit adds the missing `std::nothrow`, and makes `Thread::start()`
return `osErrorNoMemory` if allocation fails so the caller can handle
it.

Note: A case when a thread should never fail due to lack of memory
is the main thread. But the main thread's stack is a pre-allocated
array in the static memory, passed to the `Thread()` constructor
during thread creation, so it's not impacted by this change.

rtos: Remove UNITTEST macro from rtos ...

Production code should not contain any test-specific checks. Rather
than checking `UNITTEST`, unit tests can directly set
`MBED_CONF_RTOS_PRESENT=1` to make RTOS available for testing.

Note: The cellular ATHandler test also has `MBED_CONF_RTOS_PRESENT=1`
added because `ATHandler.cpp` contains a check of this variable.

Better document MemoryPool behaviour ...

Make explicit that `MemoryPool` is a raw allocator, and users need to
take care of object construction and destruction.

Change MBED_STATIC_ASSERTs version for built-in

platform: move internal headers to internal/ ...

They belong to internal folder to follow our guideline, not in source as they were.

Include mbed_assert.h in Queue

Fixed doxygen comment

Incorporating reveiw comments - Moving mbed_rtos_types.h to public headers

Moving around a few items within \rtos