[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>
3 files changed