Doc: Update and simplify Supported Platforms information

- Ubuntu 16.04 reached end-of-life, remove form supported platforms
- Add SLES 15
- Replace details of supported OS/compiler version(s) with a
  minimum supported version to avoid the info going stale before
  the release end-of-life.
- Link to qt5.git coin provisioning as it provides an always
  up-to-date information on the reference configurations.
- Clarify the statement of platforms supported by the Qt Company
  and platforms supported by Qt as a community.

Task-number: QTBUG-77795
Change-Id: Ideeb01895ed8dd37b6e708855554c99e3ce1324b
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

doc/src/getting-started/installation.qdoc

@@ -98,4 +98,19 @@ visit the following page:
 \list
 \li \l {Qt for UWP#Building from source}{Qt for UWP - Building from Source}
 \endlist
+
+\section1 Additional Information
+
+The top-level qt5 Git repository contains a set of build instructions in the
+form of \e {provisioning scripts}, used by Qt's continuous integration (CI)
+system to build and test the supported \l {Reference Configurations}. These
+scripts are useful for anyone building Qt from source, as they provide
+information on the tools and components that are required for each
+configuration.
+
+\list
+    \li \l {https://code.qt.io/cgit/qt/qt5.git/tree/coin?h=\qtver}
+           {qt/qt5.git CI provisioning for Qt \qtver}
+    \li \l {https://wiki.qt.io/CI_Overview}{CI Overview in Qt Wiki}
+\endlist
 */

doc/src/platforms/supported-platforms.qdoc

@@ -77,11 +77,17 @@
     Project. However, Qt may run on configurations other than those actively
     tested on, and additional configurations may be raised to tested state, if
     sufficient effort is made to bring continuous integration to an acceptable
-    state for that particular configuration.
+    state for that particular configuration. The reference configurations are
+    subject to change during the lifetime of a Qt release.
 
-    The Qt Company, Qt partners, open source projects and community users
-    are able to provide assistance with various different supported
-    platforms and configurations.
+    Details of the reference configurations are stored as \e {provisioning scripts}
+    in the \l {https://code.qt.io/cgit/qt/qt5.git/tree/coin?h=\qtver}
+    {qt5 Git repository}.
+
+    The Qt Company provides support for the officially supported platforms and
+    configurations. The Qt Company, Qt partners, open source projects and community
+    users are also able to provide assistance with various different platforms and
+    configurations.
 
     \section1 Availability of Packages
 

doc/src/platforms/supported-platforms.qdocinc

@@ -39,21 +39,21 @@
         \li \c x86_64
         \li \b {GCC 7}, \b {ICC}
         \li
+    \row \li SUSE Linux Enterprise Server
+        \li \c x86_64
+        \li \b {GCC 7}, GCC 8, ICC 18.x
+        \li
     \row \li Red Hat Enterprise Linux 7.x
         \li \c x86_64
-        \li \b {GCC 5.3.1}
-        \li devtoolset-4
+        \li \b {GCC 5.3.1 via devtoolset-4}
+        \li
     \row \li Ubuntu 18.04
         \li \c x86_64
-        \li GCC as provided by Canonical, \b {GCC 7.3.0}
-        \li
-    \row \li Ubuntu 16.04
-        \li \c x86_64
-        \li GCC as provided by Canonical, GCC 5.4.0
+        \li GCC as provided by Canonical, \b {GCC 7.x or later}
         \li
     \row \li Generic Linux
         \li \c x86 and \c x86_64
-        \li GCC 4.8, GCC 4.9, GCC 5, GCC 6, GCC 7
+        \li GCC (4.8 or later), ICC 18.x
         \li
 \endtable
 //! [linux]
@@ -75,7 +75,7 @@
     \row \li Windows 10
         \li \c x86 and \c x86_64
         \li \b {MSVC 2017}, \b {MSVC 2015}, \b {MinGW 7.3}
-        \li Tested on \e {Windows 10 (1809) October 2018 Update}
+        \li
     \row \li Windows 8.1
         \li \c x86 and \c x86_64
         \li MSVC 2017, MSVC 2015, MinGW 7.3
@@ -100,8 +100,8 @@
     \row \li Android 5.0 or later (that is, API Level 21 and up)
         \li \c armv7a and \c x86, \c arm64-v8 and \c x86_64
         \li \b {Clang as provided by Google}, \b {MinGW 7.3}
-        \li \b {RHEL 7.x (x86_64)}, \b{\macos 10.13}, \b {Windows 7 (x86_64)},
-            \b {Windows 10}
+        \li \b {RHEL 7.x (x86_64)}, \b{\macos 10.13}, \b {Windows 10 (x86_64)},
+            \b Windows 7
 \endtable
 //! [android]
 
@@ -131,7 +131,7 @@
     \row \li Universal Windows Platform 10
         \li \c x86, \c x86_64, and \c armv7
         \li \b{MSVC 2017}, \b{MSVC 2015}
-        \li \b{Windows 10} (1809) October 2018 Update
+        \li \b{Windows 10}
 \endtable
 
 \br