docs: task concurrency, named limits and the concurrency limits API

Rewrites the concurrency guide around the concurrency option: inline
perKey/total shapes, named limits shared across tasks with
concurrencyLimit(), trigger-time named-limit switching, and the
concurrencyLimits management namespace including pause-by-zero. The
combined concurrency limit surface (queue option, response field,
override endpoints and their reference pages) is gone; the new
concurrency-limits endpoints are documented with a reference group.

Author: matt-aitken

docs/docs.json

@@ -380,9 +380,16 @@
               "management/queues/retrieve",
               "management/queues/pause",
               "management/queues/concurrency-override",
-              "management/queues/concurrency-reset",
-              "management/queues/combined-concurrency-override",
-              "management/queues/combined-concurrency-reset"
+              "management/queues/concurrency-reset"
+            ]
+          },
+          {
+            "group": "Concurrency limits API",
+            "pages": [
+              "management/concurrency-limits/list",
+              "management/concurrency-limits/retrieve",
+              "management/concurrency-limits/override",
+              "management/concurrency-limits/reset"
             ]
           },
           {

docs/management/concurrency-limits/list.mdx

@@ -0,0 +1,4 @@
+---
+title: "List Concurrency Limits"
+openapi: "v3-openapi GET /api/v1/concurrency-limits"
+---

docs/management/concurrency-limits/override.mdx

@@ -0,0 +1,4 @@
+---
+title: "Override Concurrency Limit"
+openapi: "v3-openapi POST /api/v1/concurrency-limits/{name}/override"
+---

docs/management/concurrency-limits/reset.mdx

@@ -0,0 +1,4 @@
+---
+title: "Reset Concurrency Limit"
+openapi: "v3-openapi POST /api/v1/concurrency-limits/{name}/reset"
+---

docs/management/concurrency-limits/retrieve.mdx

@@ -0,0 +1,4 @@
+---
+title: "Retrieve Concurrency Limit"
+openapi: "v3-openapi GET /api/v1/concurrency-limits/{name}"
+---