HTML help generated always points link to documentation homepage into Supported Platforms and Configurations page

Default homepage is set as:

navigation.homepage = "Qt $QT_VER"

This of course expands to something like "Qt 5.9". When generating documentation, qdoc finds a qt-5.9 reference in Supported Platforms and Configurations page since it lists all versions there.

Offline documentation (in Qt Creator for example) is broken in this way. This works fine in our online documentation because homepage variable is overridden there to point to "Qt Documentation" page instead.

To reproduce:

• build Qt as usual
• build html documentation (make html_docs)
• open QString documentation
• try the link "Qt 5.9" in navigation bar