fix: complete Tasks GA promotion (docs, site) (#20927)

## Summary

Completes the Coder Tasks GA promotion by updating swagger tags and
regenerating API documentation and updating the frontend API structure.

## Related

Follows #20923 and #20921 which promoted Tasks from Beta/Experimental to
GA.

---

🤖 This change was written by Claude Sonnet 4.5 Thinking using
[mux](https://github.com/coder/mux) and reviewed by a human 🏂

docs/manifest.json

@@ -1166,10 +1166,6 @@
 							"title": "Enterprise",
 							"path": "./reference/api/enterprise.md"
 						},
-						{
-							"title": "Experimental",
-							"path": "./reference/api/experimental.md"
-						},
 						{
 							"title": "Files",
 							"path": "./reference/api/files.md"
@@ -1214,6 +1210,10 @@
 							"title": "Schemas",
 							"path": "./reference/api/schemas.md"
 						},
+						{
+							"title": "Tasks",
+							"path": "./reference/api/tasks.md"
+						},
 						{
 							"title": "Templates",
 							"path": "./reference/api/templates.md"

docs/reference/api/tasks.md

@@ -1,4 +1,4 @@
-# Experimental
+# Tasks
 
 ## List AI tasks