[VOL-5545] Update voltha docs

- add voltha_2.15 to release notes index toctree
- fix malformed RST table rows for VOLTHA 2.13, 2.14, 2.15 in releases page

Change-Id: I3de1d1ffcc293b7a1321613ab8f6f6bc5fbc9a0f
Signed-off-by: serkant.uluderya <serkant.uluderya@netsia.com>
diff --git a/release_notes/index.rst b/release_notes/index.rst
index b695638..0acca36 100644
--- a/release_notes/index.rst
+++ b/release_notes/index.rst
@@ -4,6 +4,7 @@
 .. toctree::
    :maxdepth: 1
 
+   voltha_2.15.rst
    voltha_2.14.rst
    voltha_2.13.rst
    voltha_2.12.rst