Use data sources instead of symlinks

Building on the work in: !1657 (closed), switched from using a filesystem feature (symlinks) to a Nanoc feature (data sources), which should allow:

• Fewer steps for folks setting up the project.
• Allow proper live updates of local changes for the first time for macOS users.

While here, did some tidy up.

Migration from symlinks

1. Go to the root dir of gitlab-docs.

2. Pull main.

3. Find if you still have any symlinks:

   find -L content/ -xtype l

   If the command returns any results, remove all of them:

   rm -f content/{ee,runner,omnibus,charts}

4. Make sure all the repositories are in the same level as gitlab-docs and they are named exactly like the following:

   • GitLab: gitlab
   • Runner: gitlab-runner
   • Omnibus: omnibus-gitlab
   • Charts: charts-gitlab

   For example, if your dev environment is under ~dev, this is how the repository names must look like:

   ~/dev/gitlab-docs
   ~/dev/gitlab
   ~/dev/gitlab-runner
   ~/dev/omnibus-gitlab
   ~/dev/charts-gitlab

5. You can now build the site like you always did https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/README.md#preview-the-documentation-website.