Link to "getting the sources" from wxGTK build instructions

This should help people who check out the repository without submodules.

It's a bit circular, as the target of the link links back to this page
itself, but doing anything else would require much deeper reorganization
that can't be done now.

Closes #23683.

docs/doxygen/overviews/install.md

@@ -49,7 +49,7 @@ about using [vcpkg][] C++ package manager for installing wxWidgets.
 Building from Source
 --------------------
 
-### Getting the sources
+### Getting the sources                {#install_get_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

docs/gtk/install.md

@@ -11,7 +11,8 @@ The simplest case                      {#gtk_simple}
 -------------------
 
 If you compile wxWidgets on Linux for the first time and don't like to read
-install instructions just do the following in wxWidgets directory:
+install instructions just [get the sources](@ref install_get_sources) and do
+the following in wxWidgets directory:
 
     $ mkdir buildgtk
     $ cd buildgtk