Add example code for the safety manual for creating safety app from the scratch

The current version of the safety manual is a bit optimistic:

https://doc-snapshots.qt.io/qtsaferenderer/qtsr-safety-project.html

If user starts to create a safety application from the scratch, the safety manual needs some additional information in the page Enabling Qt Safe Renderer in Project

See especially:

https://doc-snapshots.qt.io/qtsaferenderer/qtsr-safety-project.html#building-the-qt-safe-renderer-project

Build and Run would only work, if user has taken existing QSR example for the basis and modifies based on it.

But for the totally new projects some things must be done before the build and run in the host environment

1) The pro file should have the following info:

# Host build we are using qt for the adaptation
windows|linux|macos {
    DEFINES += QSR_HOST_BUILD
    QT += core gui
}

Otherwise it does not work. This is not needed in the target environments.

2) And there should be example for the main.cpp file as well.

See for example indicators example and its main.cpp file.

3) And for the jump start it could be explicitly mentioned, that use the e.g. indicators example: https://doc-snapshots.qt.io/qtsaferenderer/qtsaferenderer-saferenderer-telltales-example.html