Add platform's missing READMEs (elastic#82268)

* add missing readme

* update ascidoc

docs/developer/plugin-list.asciidoc

@@ -126,8 +126,8 @@ in Kibana, e.g. visualizations. It has the form of a flyout panel.
 |Utilities for building Kibana plugins.
 
 
-|{kib-repo}blob/{branch}/src/plugins/legacy_export[legacyExport]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/src/plugins/legacy_export/README.md[legacyExport]
+|The legacyExport plugin adds support for the legacy saved objects export format.
 
 
 |{kib-repo}blob/{branch}/src/plugins/management[management]
@@ -152,12 +152,12 @@ Content is fetched from the remote (https://feeds.elastic.co and https://feeds-s
 |Create choropleth maps. Display the results of a term-aggregation as e.g. countries, zip-codes, states.
 
 
-|{kib-repo}blob/{branch}/src/plugins/saved_objects[savedObjects]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/src/plugins/saved_objects/README.md[savedObjects]
+|The savedObjects plugin exposes utilities to manipulate saved objects on the client side.
 
 
-|{kib-repo}blob/{branch}/src/plugins/saved_objects_management[savedObjectsManagement]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/src/plugins/saved_objects_management/README.md[savedObjectsManagement]
+|The savedObjectsManagement plugin manages the Saved Objects management section.
 
 
 |{kib-repo}blob/{branch}/src/plugins/saved_objects_tagging_oss/README.md[savedObjectsTaggingOss]
@@ -305,8 +305,8 @@ Failure to have auth enabled in Kibana will make for a broken UI. UI-based error
 |Experimental Feature
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/cloud[cloud]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/x-pack/plugins/cloud/README.md[cloud]
+|The cloud plugin adds cloud specific features to Kibana.
 
 
 |{kib-repo}blob/{branch}/x-pack/plugins/code[code]
@@ -357,8 +357,8 @@ occuring in Kibana, initially just for the Make It Action project - alerts
 and actions.
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/features[features]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/x-pack/plugins/features/README.md[features]
+|The features plugin enhance Kibana with a per-feature privilege system.
 
 
 |{kib-repo}blob/{branch}/x-pack/plugins/file_upload/README.md[fileUpload]
@@ -374,8 +374,8 @@ or dashboards from the Kibana instance, from both server and client-side plugins
 |The GlobalSearchBar plugin provides a search interface for navigating Kibana. (It is the UI to the GlobalSearch plugin.)
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/global_search_providers[globalSearchProviders]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/x-pack/plugins/global_search_providers/README.md[globalSearchProviders]
+|The globalSearchProviders plugin provides Kibana default search providers for the GlobalSearch plugin.
 
 
 |{kib-repo}blob/{branch}/x-pack/plugins/graph/README.md[graph]

src/plugins/legacy_export/README.md

@@ -0,0 +1,3 @@
+# `legacyExport` plugin
+
+The `legacyExport` plugin adds support for the legacy saved objects export format.

src/plugins/saved_objects/README.md

@@ -0,0 +1,3 @@
+# `savedObjects` plugin
+
+The `savedObjects` plugin exposes utilities to manipulate saved objects on the client side.

src/plugins/saved_objects_management/README.md

@@ -0,0 +1,3 @@
+# `savedObjectsManagement` plugin
+
+The `savedObjectsManagement` plugin manages the `Saved Objects` management section.

x-pack/plugins/cloud/README.md

@@ -0,0 +1,3 @@
+# `cloud` plugin
+
+The `cloud` plugin adds cloud specific features to Kibana.

x-pack/plugins/features/README.md

@@ -0,0 +1,3 @@
+# `features` plugin
+
+The `features` plugin enhance Kibana with a per-feature privilege system.

x-pack/plugins/global_search_providers/README.md

@@ -0,0 +1,15 @@
+# Kibana `GlobalSearchProviders` plugin
+
+The globalSearchProviders plugin provides Kibana default search providers for the `GlobalSearch` plugin.
+
+## Server-side providers
+
+### SavedObjects
+
+This provider returns results for all saved object types that are searchable.
+
+## Client-side providers
+
+### Applications
+
+This provider returns results for the applications that are currently registered on the client-side.