[skip changelog] Add note that docs also apply to other Arduino development software (arduino#1500)

People might get the impression that, since the documentation is in the Arduino CLI repo, it only applies to Arduino CLI.
However, since Arduino CLI is used in some form in most of Arduino's development tools, much of the information is
universal.

Author: per1234

docs/index.md

@@ -1,6 +1,9 @@
 Arduino CLI is an all-in-one solution that provides Boards/Library Managers, sketch builder, board detection, uploader,
 and many other tools needed to use any Arduino compatible board and platform.
 
+In addition to being a standalone tool, Arduino CLI is the heart of all official Arduino development software (Arduino
+IDE, Arduino Web Editor). Parts of this documentation apply to those tools as well.
+
 ## Installation
 
 You have several options to install the latest version of the Arduino CLI on your system, see the [installation] page.