docs/Cluster-VictoriaMetrics.md: move Naive_cluster_scheme.png from the docs/assets/images/ folder into docs/ folder and add Cluster-VictoriaMetrics_ prefix to the image name

The docs/assets folder should be used only for assets specific to docs generation at https://docs.victoriametrics.com, e.g. css, js and images.

All the other assets related to specific docs should be placed in the same folder as the corresponding *.md file.
These assets should have the same name prefix as the corresponding doc file name. This simplifies tracking the lifetime of these assets.
For example, if the doc is removed, it is very easy to remove all assets associated with it with a simple `rm -rf docs/doc-name*` command.

This also simplifies generating correct urls for doc-specific assets from both https://docs.victoriametrics.com
and from https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/docs/ - just refer to the asset name without any directory prefixes.

docs/Cluster-VictoriaMetrics.md

@@ -37,7 +37,7 @@ Each service may scale independently and may run on the most suitable hardware.
 This is a [shared nothing architecture](https://en.wikipedia.org/wiki/Shared-nothing_architecture).
 It increases cluster availability, and simplifies cluster maintenance as well as cluster scaling.
 
-![Naive cluster scheme](assets/images/Naive_cluster_scheme.png)
+![Naive cluster scheme](Cluster-VictoriaMetrics_cluster-scheme.png)
 
 ## Multitenancy