Add new "Installation" page to the manual

Currently this simply links to the existing port-specific installation
instructions, but this should provide a less confusing and more
prominent entry point than the existing topics.

docs/doxygen/mainpages/topics.h

@@ -18,9 +18,9 @@ topics related to building applications with wxWidgets.
 
 @section page_topics_starting Starting with wxWidgets
 
+@li @subpage overview_install
 @li @subpage overview_helloworld
 @li @subpage overview_roughguide
-@li @subpage overview_cmake
 @li @subpage overview_referencenotes
 
 @section page_topics_programming Conceptual Overviews

docs/doxygen/overviews/helloworld.h

@@ -14,6 +14,9 @@
 This page shows a very simple wxWidgets program that can be used as a skeleton
 for your own code. While it does nothing very useful, it introduces a couple of
 important concepts and explains how to write a working wxWidgets application.
+Trying building and running this application is also a good way of checking
+that wxWidgets is correctly installed on your system. And if you haven't
+installed wxWidgets yet, please @ref overview_install "do it first".
 
 Note that this simple example creates the UI entirely from C++ code which is
 fine for a simple example, but more realistic examples will typically define

docs/doxygen/overviews/install.md

@@ -0,0 +1,81 @@
+Installing wxWidgets    {#overview_install}
+====================
+
+wxWidgets headers and libraries must be available in order to build the
+applications using it, so the first step when starting to use wxWidgets is to
+install it. This can be done, for all platforms and library versions, by
+building wxWidgets from sources, but for the most common platforms pre-built
+binaries of the libraries are also provided, so the first decision to make is
+whether to use these binaries or build the library yourself. Building the
+libraries yourself allows you to compile the latest version using exactly the
+options you need, and so is the most flexible solution, but using the binaries
+may be simpler and faster -- the choice is yours, just follow the instructions
+in the corresponding section below depending on the approach you prefer. Of
+course, you only need to do one _or_ the other, _not_ both.
+
+
+Using Binaries
+--------------
+
+How to install binaries depends on your platform:
+
+- For Microsoft Windows (MSW), wxWidgets project provides official binaries
+  on the [Downloads page](http://www.wxwidgets.org/downloads/), please see
+  the [instructions for using them](@ref plat_msw_binaries).
+
+- For Linux, and other free Unix-like systems, wxWidgets packages are available
+  in system repositories under the name "wxGTK". Note that to develop
+  applications using wxWidgets you may need to install the "development"
+  packages and not just the libraries needed for running the applications using
+  wxWidgets. For example, under Debian and Debian-derived systems such as
+  Ubuntu, you need to run `apt get libwxgtkX.Y-dev`.
+
+- For macOS, wxWidgets is available in third-party package managers such as
+  [brew][1] or [MacPorts][2], and you can install them in the usual way.
+
+[1]: https://brew.sh/
+[2]: https://www.macports.org/
+
+
+Additionally, some third-party C++ package managers also provide wxWidgets
+binaries. For example, please see [this post][3] for the instructions about
+using [vcpkg][4] C++ package manager for installing wxWidgets.
+
+[3]: https://www.wxwidgets.org/blog/2019/01/wxwidgets-and-vcpkg/
+[4]: https://github.com/microsoft/vcpkg
+
+
+Building from Source
+--------------------
+
+### Getting the sources
+
+To build the library you need to get its sources first. The recommended way to
+do it is to use Git to check them out from the official wxWidgets repository
+using the following command:
+
+    $ git clone --recurse-submodules https://github.com/wxWidgets/wxWidgets.git
+
+Alternatively, you can download the sources from the [downloads page][1].
+Please note that all the source archives in different formats (ZIP, 7z,
+tar.bz2) contain the same files, but use different line ending formats: Unix
+("LF") for the latter one and DOS ("CR LF") for the two other ones, and it is
+usually preferable to choose the format corresponding to the current platform.
+When downloading the sources with DOS ends of lines, prefer 7z format for much
+smaller file size.
+
+
+### Selecting the build system
+
+wxWidgets can be built using [CMake](https://cmake.org/) under all platforms.
+Please follow [CMake build instructions](@ref overview_cmake) if you prefer to
+use it.
+
+Otherwise, please use the appropriate instructions depending on your platform:
+
+- For native wxMSW port under [Microsoft Windows](@ref plat_msw_install).
+- For wxGTK under [Linux and other Unix-like systems](@ref plat_gtk_install).
+- For native wxOSX port under [macOS](@ref plat_osx_install).
+
+The wxWidgets ports mentioned above are the main ones, however other variants
+also exist, see [platform details](@ref page_port) page for the full list.