Removing CLI plugins API documentation from official documentation as it is now deprecated.

Adding depreciation notice.

Signed-off-by: Silvin Lubecki <silvin.lubecki@docker.com>

Author: silvin-lubecki

cli-plugins/manager/metadata.go

@@ -10,7 +10,7 @@ const (
 	MetadataSubcommandName = "docker-cli-plugin-metadata"
 )
 
-// Metadata provided by the plugin. See docs/extend/cli_plugins.md for canonical information.
+// Metadata provided by the plugin.
 type Metadata struct {
 	// SchemaVersion describes the version of this struct. Mandatory, must be "0.1.0"
 	SchemaVersion string `json:",omitempty"`

docs/deprecated.md

@@ -50,6 +50,7 @@ The table below provides an overview of the current status of deprecated feature
 
 Status     | Feature                                                                                                                            | Deprecated | Remove
 -----------|------------------------------------------------------------------------------------------------------------------------------------|------------|------------
+Deprecated | [CLI plugins support](#cli-plugins-support)                                                                                        | v20.03.0   | -
 Deprecated | [Pushing and pulling with image manifest v2 schema 1](#pushing-and-pulling-with-image-manifest-v2-schema-1)                        | v19.03.0   | v20.03.0
 Removed    | [`docker engine` subcommands](#docker-engine-subcommands)                                                                          | v19.03.0   | v20.03.0
 Removed    | [Top-level `docker deploy` subcommand (experimental)](#top-level-docker-deploy-subcommand-experimental)                            | v19.03.0   | v20.03.0
@@ -87,6 +88,11 @@ Removed    | [`--api-enable-cors` flag on `dockerd`](#--api-enable-cors-flag-on-
 Removed    | [`--run` flag on `docker commit`](#--run-flag-on-docker-commit)                                                                    | v0.10.0    | v1.13.0
 Removed    | [Three arguments form in `docker import`](#three-arguments-form-in-docker-import)                                                  | v0.6.7     | v1.12.0
 
+### CLI plugins support
+
+**Deprecated in Release: v20.03.0**
+
+CLI Plugin API is now marked as deprecated.
 
 ### Pushing and pulling with image manifest v2 schema 1