[skip changelog] Document packages[*].platforms[*].deprecated package index field (#1510)

In the event an Arduino boards platform is deprecated by the maintainer, it will be useful to clearly communicate this
fact to the user and ensure they will easily find a preferred alternative platform. In order to facilitate this, a
`packages[*].platforms[*].deprecated`field was added to the Arduino package index format and this information is used to
enhance the user interfaces of the official Arduino development software.

This useful feature is now documented in the Arduino package index specification.

Author: per1234

docs/package_index_json-specification.md

@@ -234,6 +234,9 @@ Each PLATFORM describes a core for a specific architecture. The fields needed ar
 - `architecture`: is the architecture of the platform (avr, sam, etc...). It must match the architecture of the core as
   explained in the [Arduino platform specification](platform-specification.md#hardware-folders-structure)
 - `version`: the version of the platform.
+- `deprecated`: (optional) setting to `true` causes the platform to be moved to the bottom of all Boards Manager and
+  [`arduino-cli core`](https://arduino.github.io/arduino-cli/latest/commands/arduino-cli_core/) listings and marked
+  "DEPRECATED".
 - `category`: this field is reserved, a 3rd party core must set it to `Contributed`
 - `help`/`online`: is a URL that is displayed on the Arduino IDE's Boards Manager as an "Online Help" link
 - `url`, `archiveFileName`, `size` and `checksum`: metadata of the core archive file. The meaning is the same as for the