Add terminology-describing doc

[bocekm]

Describe what we mean by:

• leapp package,
• leapp-related packages (leapp, leapp-deps, snactor, python2-leapp, python3-leapp),
• Leapp project-related packages (leapp, leapp-deps, snactor, python2-leapp, python3-leapp, leapp-repository, leapp-repository-deps, leapp-repository-data),
• leapp-repository package,
• leapp-repository-related packages (leapp-repository, leapp-repository-deps, leapp-repository-data).
• Leapp repository (not leapp-repository) - can point to https://leapp.readthedocs.io/en/latest/terminology.html#repository