Unification of build and install steps

Currently, in 5.12, only the Neptune docs explains steps how to build the key components: https://doc.qt.io/Neptune3UI/neptune3ui-build-run.html

A new commit with the new landing page mentions that steps on the landing page now as well, still the problem remains that QtIVI, appman and Neptune explain steps for building and installing it in different ways. Moreover, QtIVI only explains configuring it. Even if the user would just "qmake-make" it, it is not said, it should be installed with "make install"

Even though, it is might be clear for experienced developers where components are installed, it should be explained at least for key ones where they are. For example, Neptune only says "Run the following command in a terminal under the neptune3-ui installation directory", whereas "installation directory" is unknown. Not mentioning the fact that the command "./neptune3-ui " implies the the Neptune is started from the current directory. Plus the installation step is not mentioned before, which makes "installation directory" irrelevant.