[skip changelog] Link to Sketch build process docs from Platform specification (arduino#1025)

The "Build process" section of the Arduino platform specification contains a short summary of the Arduino sketch build
process. The "Sketch build process" page of the documentation provides much more detailed information, which wouldn't be
appropriate in the platform specification, but could well be of interest to a platform developer.

Adding a link to the "Sketch build process" documentation from that section of the Platform specification makes all
relevant information easily accessible to the reader.

Author: per1234

docs/platform-specification.md

@@ -124,9 +124,9 @@ unused, it is reserved for future use (probably together with the Boards Manager
 
 ### Build process
 
-The platform.txt file is used to configure the build process. This is done through a list of **recipes**. Each recipe is
-a command line expression that explains how to call the compiler (or other tools) for every build step and which
-parameter should be passed.
+The platform.txt file is used to configure the [build process](sketch-build-process.md). This is done through a list of
+**recipes**. Each recipe is a command line expression that explains how to call the compiler (or other tools) for every
+build step and which parameter should be passed.
 
 The Arduino development software, before starting the build, determines the list of files to compile. The list is
 composed of: