Document TarMK design

We should improve our documentation of the internal design of the the TarMK. There is currently a single section.

• Add a high level class diagram and description of the overall structure of the TarMK.
• Decide what to do with segmentmk.md. My preference would be to incorporate everything from it we didn't cover so far into segment/overview.md, segment/records.md and segment/tar.md.
• Rewrite, clarify the design section in segment/overview.md.