Document on how to deploy contents of a GitLab repository to an EOS folder

Context: Recently Ismael configured automatic deployment from GitLab for the site webcast-lite-qa, basing on the existing project https://gitlab.cern.ch/ci-tools/ci-web-deployer . Already today we have site owners who manage their websites' files in GitLab and copy them automatically to their EOS location.

The idea is to provide a recipe for setting up such automated deployment of content from user's GitLab repository to the EOS location of their choice (provided they own an account with write privileges on that EOS location).