Add releasenotes and changelog for 15.5 (opencast#5854)

### Adds the changelog and release notes for Opencast 15.5

* [x] have a concise title
* [ ] [close an accompanying
issue](https://docs.opencast.org/develop/developer/#participate/development-process/#automatically-closing-issues-when-a-pr-is-merged)
if one exists
* [x] [be against the correct
branch](https://docs.opencast.org/develop/developer/development-process#acceptance-criteria-for-patches-in-different-versions)
* [ ] include migration scripts and documentation, if appropriate
* [x] pass automated tests
* [x] have a clean commit history
* [x] [have proper commit messages (title and body) for all
commits](https://medium.com/@steveamaza/e028865e5791)

docs/guides/admin/docs/changelog.md

@@ -4,6 +4,13 @@ Changelog
 Opencast 15
 -----------
 
+### Opencast 15.5
+
+- [[#5757](https://github.com/opencast/opencast/pull/5757)] -
+  Add slide text and segments to Tobira harvest API
+- [[#5685](https://github.com/opencast/opencast/pull/5685)] -
+  Paella 7 mp4 quality selector
+
 ### Opencast 15.4
 
 - [[#5738](https://github.com/opencast/opencast/pull/5738)] -

docs/guides/admin/docs/releasenotes.md

@@ -1,5 +1,14 @@
 # Opencast 15: Release Notes
 
+## Opencast 15.5
+
+### Improvements
+
+- **Add slide text and segments to Tobira harvest API**
+[[#5757](https://github.com/opencast/opencast/pull/5757)]
+- **Paella 7 mp4 quality selector**
+[[#5685](https://github.com/opencast/opencast/pull/5685)]
+
 ## Opencast 15.4
 
 ### Bug Fixes