Add docs page on "data"

[UJehle]

Goal of this issue

Create a docs page on data based on the content which previously was under the "technical architecture" section (needs an update!).

Deliverables

New docs page on data. Find good balance between openness and transparency. Provide overview, not overshare. Maybe also add a graphic.

Branch to derive

p4b-website/development

Steps

• [ ] create content & bring it to GitHub (Dilan)
• [ ] check (UJ)
• [ ] translation (UJ)
• [ ] add link to this section in GOAT (Dilan)