* Project Configuration Guide: how to configure the project, purpose of
sdkconfig.defaults, where is the configuration stored...
* Component configuration: how to write Kconfig files, how to ensure
backward compatibility...
Also, general information about Kconfig and its files was added. Other parts of the
documentation (e.g. build-system) now point to these new guides in order
to provide more info.
* reverted old faulty mechanism to set different
standard according to the toolchain
* Using -std=gnu++2b now for both gcc and clang
* Added a build test app to check the C++ standard in IDF
* Updated english docs to reflect the change to C++23
