added Ditto release notes for version 2.3.1

Signed-off-by: Thomas Jaeckle <thomas.jaeckle@bosch.io>

documentation/src/main/resources/_data/sidebars/ditto_sidebar.yml

@@ -23,6 +23,9 @@ entries:
       - title: Release Notes
         output: web
         folderitems:
+          - title: 2.3.1
+            url: /release_notes_231.html
+            output: web
           - title: 2.3.0
             url: /release_notes_230.html
             output: web

documentation/src/main/resources/pages/ditto/release_notes_221.md

@@ -16,7 +16,7 @@ Compared to the latest release [2.2.0](release_notes_220.html), the following bu
 ### Bugfixes
 
 This is a complete list of the
-[merged pull requests](https://github.com/eclipse/ditto/pulls?q=is%3Apr+milestone%3A2.2.0), including the fixed bugs.
+[merged pull requests](https://github.com/eclipse/ditto/pulls?q=is%3Apr+milestone%3A2.2.1), including the fixed bugs.
 
 #### [Update logback to 1.2.8 due to "possibility of vulnerability"](https://github.com/eclipse/ditto/pull/1253)
 

documentation/src/main/resources/pages/ditto/release_notes_231.md

@@ -0,0 +1,38 @@
+---
+title: Release notes 2.3.1
+tags: [release_notes]
+published: true
+keywords: release notes, announcements, changelog
+summary: "Version 2.3.1 of Eclipse Ditto, released on 31.01.2022"
+permalink: release_notes_231.html
+---
+
+This is a bugfix release, no new features since [2.3.0](release_notes_230.html) were added.
+
+## Changelog
+
+Compared to the latest release [2.3.0](release_notes_230.html), the following bugfixes were added.
+
+### Bugfixes
+
+This is a complete list of the
+[merged pull requests](https://github.com/eclipse/ditto/pulls?q=is%3Apr+milestone%3A2.3.1), including the fixed bugs.
+
+#### [Fix that placeholder `time:now` could not be used in connection header mapping](https://github.com/eclipse/ditto/pull/1292)
+
+The in version 2.3.0 newly added placeholder `time:now` could not be used in connectivity's header mapping, 
+this is now fixed.
+
+#### [Enable use of entity placeholders for MQTT and HTTP connections](https://github.com/eclipse/ditto/pull/1293)
+
+The `entity:id` placeholder could not be used in MQTT and HTTP connections, this is now fixed.
+
+#### [Reduce the likelihood of search index inconsistency due to reordering of patch updates](https://github.com/eclipse/ditto/pull/1296)
+
+This fix speeds up the consistency of search entries which could get inconsistent during rolling updates for very active 
+things. 
+
+#### [Fixed that JSON `null` in "correlation-id" of Ditto Protocol headers were parsed as JSON String "null"](https://github.com/eclipse/ditto/pull/1295)
+
+When parsing messages as Ditto Protocol, a `"correlation-id"` header field being `null` was translated to the string 
+literal `"null"` - this has been fixed by treating the "correlation-id" as not being present instead.