Make the focus of episode 04 on archiving

[hlapp]

The permanence of archiving, and the digital availability of archived records, directly influences reproducibility (input data or notebook inaccessible means no reproduction possible). This is discussed in the first part of episode 04 (albeit under a different title), and should be its sole focus (including in the title).

Eventually, this episode should probably be absorbed into the Using Github for publishing/sharing notebooks lesson that is in development (see #34 and #33).