[MetaROR] Final formatting, consistency & referencing (pre-release)

[LukasWallrich]

Formatting, consistency and referencing tasks to complete before releasing the next version. Some can be agent-assisted but each needs a human review pass.

• Use "this handbook" consistently — the text currently alternates between "paper", "article" and "handbook". This needs context judgement (not every "paper" refers to the handbook), so review each change rather than running a blind find-and-replace. (R2)
• Add introductory lines at the beginning of each section to orient casual readers. (R2)
• Final referencing pass — fix broken citations and verify DOIs, citation metadata and reference completeness across the handbook before release.

Related existing issues (whoever takes this owns these too):

• Broken citations #3 Broken citations

Handled in other issues:

• Add date-accessed timestamps to all web references → Automatable tasks
• Conceptual terminology/typology alignment → Cross-cutting editorial changes