Documentation: Restructure staging workflow description

[fricklerhandwerk]

Problem

The section structure does not introduce concepts in a meaningful order. Details:

#225920 (review)

Proposal

@NixOS/documentation-team Implement suggestions in the linked comment.

Checklist

• checked latest Nixpkgs manual (source) and latest NixOS manual (source)
• checked open documentation issues for possible duplicates
• checked open documentation pull requests for possible solutions

[infinisil]

In #245243 I effectively rewrote those parts of the contribution manual. There's now a dedicated section to explain which branch should be used here (linked to from a new section on how to make a pull request here), and a section explaining how merged PR's propagate here.

It's not perfect, but it's better than before, and maybe good enough for this to be closable.