New guide chapter: Efficient Metadata Handling

[psss]

Discussed yesterday with @lukaszachy that it would be nice to summarize all features which are available for efficient metadata handling and preventing data duplication. The new guide chapter should cover these:

• using fmf inheritance
• adjust based on context
• yaml referencing features
• sharing tests across repositories

[psss]

Simple example demonstrating yaml referencing:

discover:
  - name: upstream
    how: fmf
    filter: &selection tier:1 & tag:container
    url: https://github.com/teemtee/tmt
  - name: fedora
    how: fmf
    filter: *selection
    url: https://src.fedoraproject.org/rpms/tmt/