Move task manager README.md to root of plugin (#82012) (#82048)

* Move task manager README.md to root of plugin

* Fix failing test, update task manager plugin description in docs

Author: mikecote

docs/developer/plugin-list.asciidoc

@@ -488,8 +488,8 @@ the alertTypes by the Stack in the alerting plugin, register associated HTTP
 routes, etc.
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/task_manager[taskManager]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/x-pack/plugins/task_manager/README.md[taskManager]
+|The task manager is a generic system for running background tasks.
 
 
 |{kib-repo}blob/{branch}/x-pack/plugins/telemetry_collection_xpack/README.md[telemetryCollectionXpack]

x-pack/plugins/task_manager/server/README.md renamed to x-pack/plugins/task_manager/README.md

@@ -1,7 +1,8 @@
 # Kibana task manager
 
-The task manager is a generic system for running background tasks. It supports:
+The task manager is a generic system for running background tasks.
 
+It supports:
 - Single-run and recurring tasks
 - Scheduling tasks to run after a specified datetime
 - Basic retry logic