doc: develop: Improve C++ language support documentation

This commit reworks the C++ language support documentation such that:

1. it contains more up-to-date information regarding the available
   features.

2. it makes a clear distinction between the Zephyr C++ subsystem and
   the C++ standard library and describe the functionalities they
   provide.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>