Previews: beta docs (#47993)

* #44449: preview docs draft

* #44449: remove localhost

* #44449: fixes

* #44449: remove localhost

* Update docs/sources/dashboards/previews.md

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* Update docs/sources/dashboards/previews.md

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* Update docs/sources/dashboards/previews.md

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* Update docs/sources/dashboards/previews.md

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* Update docs/sources/dashboards/previews.md

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* Update docs/sources/dashboards/previews.md

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* Update docs/sources/dashboards/previews.md

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* Update docs/sources/dashboards/previews.md

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* Update docs/sources/dashboards/previews.md

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* Update docs/sources/dashboards/previews.md

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* Update docs/sources/dashboards/previews.md

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* #44449: update link

* #44449: review comment https://github.com/grafana/grafana/pull/47993#discussion_r866363311

* #44449: https://github.com/grafana/grafana/pull/47993#discussion_r866363825

* #44449: https://github.com/grafana/grafana/pull/47993#discussion_r866364182

* #44449: https://github.com/grafana/grafana/pull/47993#discussion_r866365003

* #44449: https://github.com/grafana/grafana/pull/47993#discussion_r866359611

* #44449: https://github.com/grafana/grafana/pull/47993#discussion_r866361949

* #44449: add note about the first crawler run

* #44449: https://github.com/grafana/grafana/pull/47993/files#r866355629

* Update docs/sources/dashboards/previews.md

Co-authored-by: JitaC <70489351+achatterjee-grafana@users.noreply.github.com>

* fix up

* Update docs/sources/dashboards/previews.md

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* typo

* Update docs/sources/dashboards/previews.md

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* Update docs/sources/dashboards/previews.md

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* rephrase

* replace gifs with webms

* reword

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docs/sources/image-rendering/_index.md

@@ -96,6 +96,29 @@ RENDERING_CLUSTERING_TIMEOUT=30
 }
 ```
 
+##### Cluster mode `contextPerRenderKey` (experimental)
+
+> **Note:** This feature is available in Image Renderer v3.4.0 and later versions.
+
+In `contextPerRenderKey` mode, the plugin will reuse the same [browser context](https://chromedevtools.github.io/devtools-protocol/tot/Target/#method-createBrowserContext) for all rendering requests sharing the same `renderKey` auth cookie and target domain within a short time window. Each new request will open a new page within the existing context. Contexts are closed automatically after 5s of inactivity.
+
+In the case of `contextPerRenderKey` mode, the `clustering.max_concurrency` option refers to the number of open contexts rather than the number of open pages. There is no way to limit the number of open pages in a context.
+
+`contextPerRenderKey` was designed to improve the performance of the [dashboard previews crawler]({{< relref "../dashboards/previews.md#about-the-dashboard-previews-crawler" >}}).
+
+```json
+{
+  "rendering": {
+    "mode": "clustered",
+    "clustering": {
+      "mode": "contextPerRenderKey",
+      "maxConcurrency": 5,
+      "timeout": 30
+    }
+  }
+}
+```
+
 #### Reusable (experimental)
 
 When using the rendering mode `reusable`, one browser instance will be created and reused. A new incognito page will be opened for each request. This mode is experimental since, if the browser instance crashes, it will not automatically be restarted. You can achieve a similar behavior using `clustered` mode with a high `maxConcurrency` setting.