docs: remove experimental chat API from generated docs (#22897)

The chat API is experimental (behind `ExperimentAgents`) and not ready
for public documentation yet. This removes swagger annotations from the
chat handlers so they no longer appear in the generated API reference at
https://coder.com/docs/reference/api/chats.

## Changes
- Remove `@swagger` annotations from 5 chat handlers in
`coderd/chats.go`
- Regenerate `coderd/apidoc/swagger.json` and `docs.go`
- Delete `docs/reference/api/chats.md`
- Remove Chats entry from `docs/manifest.json`

docs/manifest.json

@@ -1406,10 +1406,6 @@
 							"title": "Builds",
 							"path": "./reference/api/builds.md"
 						},
-						{
-							"title": "Chats",
-							"path": "./reference/api/chats.md"
-						},
 						{
 							"title": "Debug",
 							"path": "./reference/api/debug.md"