Remove redundant and overly detailed per-platform installation instructions

The main Getting started with Qt has all the info and we should just
link to that.

Change-Id: I5cb5d3b3276989f06ad1fb7726d121928fbab22a
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Mats Honkamaa <mats.honkamaa@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

doc/src/platforms/linux.qdoc

@@ -33,16 +33,8 @@
 
     Qt's support for different Linux platforms is extensive and mature.
 
-    \section1 Downloading and Installing Qt
-
-    There are two ways to install Qt:
-    \list 1
-    \li through the Qt Installers - downloads and installs Qt
-    \li through the \e{Qt sources}.
-    \endlist
-
-    You can download the Qt 5 installers and sources from the \l Downloads page.
-    For more information, visit the \l{Getting Started with Qt} page.
+    To download and install Qt for Linux, follow the instructions on the
+    \l{Getting Started with Qt} page.
 
     \section2 Requirements for Development Host
 

doc/src/platforms/macos.qdoc

@@ -39,23 +39,8 @@
     Application development on \macos is done using Xcode, which is available
     from \l{https://developer.apple.com/xcode/}.
 
-    \section1 Downloading and Installing Qt
-
-    There are two ways to install Qt:
-    \list 1
-    \li through the \e{Qt Installers} - downloads and installs Qt
-    \li through the \e{Qt sources}.
-    \endlist
-
-    You can download the Qt 5 installers and sources from the \l Downloads page.
-    For more information, visit the \l{Getting Started with Qt} page.
-
-    \section2 Building Qt 5 from Source
-
-    Below, you will find more information about building Qt from source.
-    \list
-        \li \l{Qt for macOS - Building from Source} - building and installing from source
-    \endlist
+    To download and install Qt for \macos, follow the instructions on the
+    \l{Getting Started with Qt} page.
 
     \note Qt 5 uses Cocoa, therefore, building for Carbon is not possible.
 

doc/src/platforms/windows.qdoc

@@ -40,28 +40,8 @@
     page contains a list of Windows versions and compilers tested to work with
     Qt.
 
-    \section1 Downloading and Installing Qt
-
-    There are two ways to install Qt:
-    \list 1
-    \li through the Qt Installers - downloads and installs Qt
-    \li through the \e{Qt sources}.
-    \endlist
-
-    You can download the Qt 5 installers and sources from the \l Downloads page.
-    For more information, visit the \l{Getting Started with Qt} page.
-
-    \section2 Building Qt 5 from Source
-
-    You can also build Qt 5 from the source package and configure it according
-    to your target platform. The source packages are obtained from
-    \l{http://www.qt.io/download/}.
-
-    Below, you will find more information about building Qt from source.
-    \list
-        \li \l{Qt for Windows - Requirements}
-        \li \l{Qt for Windows - Building from Source}
-    \endlist
+    To download and install Qt for Windows, follow the instructions on the
+    \l{Getting Started with Qt} page.
 
     \section1 Deployment and Other Issues