docs: Update documentation on building with openharmony support

Update documentation on building with openharmony support after
eclipse/oniro-core/oniro!250
and
https://gitlab.eclipse.org/eclipse/oniro-core/meta-openharmony/-/merge_requests/65


have landed

Signed-off-by: Bernhard Rosenkränzer <bernhard.rosenkraenzer.ext@huawei.com>