docs: move changelog to dir (#6853)

Moving changelog-related docs to a separate dir should make it easier to
navigate in `docs/` folder.

-----------

The change shouldn't have any visual changes or changes to the links.

Signed-off-by: hagen1778 <roman@victoriametrics.com>

docs/changelog/CHANGELOG.md

@@ -1,14 +1,3 @@
----
-weight: 100
-title: CHANGELOG
-menu:
-  docs:
-    identifier: vm-changelog
-    parent: victoriametrics
-    weight: 100
-aliases:
-  - /CHANGELOG.html
----
 The following `tip` changes can be tested by building VictoriaMetrics components from the latest commits according to the following docs:
 
 * [How to build single-node VictoriaMetrics](https://docs.victoriametrics.com/#how-to-build-from-sources)

docs/changelog/_index.md

@@ -0,0 +1,12 @@
+---
+weight: 100
+title: CHANGELOG
+menu:
+  docs:
+    identifier: vm-changelog
+    parent: victoriametrics
+    weight: 100
+aliases:
+  - /CHANGELOG.html
+---
+{{% content "CHANGELOG.md" %}}