Revert "try to remove assets folder"

This reverts commit 251d3d54a4.

Reason for revert: this commit removes important information, which was placed in the docs/assets folder
in order to reduce the probability of putting images related to particular docs here,
with the reasoning why this is a bad practice.

This information should remain in the docs/assets folder. Probably, the file should be renamed
from README.md to README or to some other visible name, which doesn't lead to generation of unneded
documentation for the assets folder.

docs/assets/README.md

@@ -0,0 +1,12 @@
+This folder must contain only files, which are needed for generating https://docs.victoriametrics.com .
+
+This folder **should not contain** files specific for a particular documentation pages such as images
+used in a particular doc. Such files must be placed in the same folder as the doc itself
+and they must have the same prefix as the doc filename. For example, all the images for docs/foo/bar.md
+should have filenames starting from docs/foo/bar. This simplifies lifetime management for these files.
+For example, if the corresponding doc is removed, then it is easy to remove all the associated
+images with a simple `rm -rf docs/foo/bar*` command. This also simplifies referring the associated images
+from docs displayed at various views:
+
+- https://docs.victoriametrics.com
+- https://github.com/VictoriaMetrics/VictoriaMetrics/tree/master/docs