doc(ci): adding constraint on the branch name

Karim Harouat made 2 merge requests with a source branch name scheme like username/my_branch_name.

It resulted on the docker push to the private registry:

See https://jenkins-oai.eurecom.fr/job/RAN-Ubuntu18-Image-Builder/3433/console

[2023-01-03 08:58:28,686]    DEBUG:  docker push porcepix.sboai.cs.eurecom.fr/oai-enb:kharobcom/fixb
buildhelper-f277a27f
invalid reference format
****@obelix:~
[2023-01-03 08:58:28,686]    ERROR: Could not push oai-enb to local registry : porcepix.sboai.cs.eurecom.fr/oai-enb:kharobcom/fixbuildhelper-f277a27f
[2023-01-03 08:58:28,842]    ERROR: Scenario failed 1 time(s)
Failed Scenario

I also modified the wiki page: https://gitlab.eurecom.fr/oai/openairinterface5g/-/wikis/policy/branch-policy