feat(docs): update release notes before rls2306

Some descriptions will be updated later according to the items below:
- Some issues (mostly RCAs) need more investigation.
- Some issues may have been fixed, waiting for VPP coverage results.
- Some issues may have become fixable on CSIT side.
  E.g. NAT tput tests can use higher ramp-up rate,
  as the decommissioned 2n-skx testbeds are no longer a bottleneck.

Change-Id: I3a98fb308908eeee71cababb5a197413cd232e13
Signed-off-by: Vratko Polak <vrpolak@cisco.com>