Update NETCONF user guide
User guide still contains references to removed bierman-02 paths:
`/restconf/config`, `/restconf/operational` and `restconf/operations`.
Fix them according to RFC 8040.
Make other small changes in order to make documentation up-to-date
with current state: update RFC-7895 link, update requests, default
ports and the netopeer2 usage.
JIRA: NETCONF-946
Change-Id: I94cd1fdf53ff3038a691f43f615d96645df7f023
Signed-off-by: Yaroslav Lastivka <yaroslav.lastivka@pantheon.tech>
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>