Discourage use of legacy API in documentation

The original design of the action was extremely limiting and so the API was expanded to allow the action to become a flexible, general purpose tool for compilation testing of Arduino sketches. The old API continues to be supported, but deprecated. Toward the goal of discouraging the use of the old API in new workflows, the documentation should only mention the new API, or if it is necessary to mention the old API (as is the case with the `libraries` input due to the significant differences in behavior when the new API is used vs. the old), it should be clearly stated that this is deprecated.

Author: per1234

README.md

@@ -11,10 +11,8 @@ The version of [Arduino CLI](https://github.com/arduino/arduino-cli) to use. Def
 ### `fqbn`
 
 The fully qualified board name to use when compiling. Default `"arduino:avr:uno"`.
-For 3rd party boards, also specify the Boards Manager URL:
-```yaml
-  fqbn: '"sandeepmistry:nRF5:Generic_nRF52832" "https://sandeepmistry.github.io/arduino-nRF5/package_nRF5_boards_index.json"'
-```
+
+If the board is from one of the platforms provided by Arduino's [default package index](https://downloads.arduino.cc/packages/package_index.json), the board's platform dependency will be automatically detected and the latest version installed. For boards of platforms not in the default package index, previous versions, or other platform sources, the platform dependency must be defined via the [`platforms` input](#platforms).
 
 ### `platforms`
 
@@ -71,7 +69,7 @@ Default `"- source-path: ./"`. This causes the repository to be installed as a l
 
 Libraries are installed under the Arduino user folder at `~/Arduino/libraries`.
 
-Note: the original space-separated list format is also supported. When this syntax is used, the repository under test will always be installed as a library.
+Note: when the deprecated space-separated list format of this input is used, the repository under test will always be installed as a library.
 
 #### Sources: