create default gitlabCI.yaml file for lektor websites

As we are moving out of jenkins to gitlabCI, we need to create a default gitlabCI file for our lektor websites.

The integration with gitlabCI will also help us to review merge requests, as merge requests should get results and appear under users gitlab 'pages'.

First version was made by Gus at support$89

I have made some changes and they are at https://gitlab.torproject.org/tpo/manual/-/blob/translations/.gitlab-ci.yml

There are still some unresolved issues:

• The translations are not complete
• The packages are not the same we use on production, especially lektor and lektor-i18n package
• reduce the traffic w/docker, maybe have local images ready
• test before build
• stop spamming tor-bots (it is sending a minimum of 3 messages per event)
• the gitlabCI.yaml file should be configured in a way that allows contributors without commit rights on the repo to push a branch to their own fork and be able to see the build on their own 'pages'. This will allow them to preview their changes before submitting the merge request, without the need to install lektor and build it in local.