Resolve "Carry over some documentation from the Utopia docs"

This MR updates the documentation by carrying over parts from the Utopia documentation, generalising and extending them in the process.

Can this MR be accepted?

• Documentation extended or updated
  • Data handling:
    • Add page on handling large amounts of data, proxies, dask
    • Example load configurations for DataManager (do this after !135 (merged) is merged)
  • Page on multidimensional data generation, loading, handling, plotting (will be done as #133 (closed))
  • Plotting:
    • Add page on ExternalPlotCreator and its features
      • Extend PlotHelper section there (@jeremiastraub)
      • Add Animations section
    • Put plot creator docs into their own folder
    • Add page on parameter sweep plots
  • Make more entries visible on main index page and put FAQ to the bottom
• No relevant code changes; no changes in user-facing behaviour
  • Mostly whitespace changes
  • Deleted some unused imports (have my linter configured now to show me those things 🙈 )
  • Removed nesting in _pcr_ext_modules, placing the modules as private modules in a more convenient place

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved by @jeremiastraub
  • Reviewed & approved by @blsqr

Related issues

Closes #130 (closed)

Meta task: #117 (closed)