Standardize docs shell on PyData Sphinx Theme

[cmccomb]

Summary

Move design-research-experiments to the PyData Sphinx Theme and align its docs shell with the shared design-research documentation baseline.

Why

This repo already has useful study-structure and examples material, but it still uses the RTD theme and should share the same top-level docs pattern as the sibling libraries.

Proposed work

• Replace sphinx_rtd_theme with pydata-sphinx-theme.
• Standardize the top-level docs structure around the common surfaces we want across repos:
  • overview / landing page
  • installation / dependencies
  • quickstart
  • examples / recipes
  • concepts / study structure / workflows
  • CLI reference when applicable
  • API reference
• Make the landing experience clearer for users deciding whether they are here to define studies, run experiments, or inspect outputs.
• Preserve the experiment-specific content while fitting it into the shared navigation baseline.

Acceptance criteria

• docs/conf.py uses pydata_sphinx_theme.
• The top-level docs nav aligns with the shared baseline used across the module repos.
• Examples / recipes, study structure, CLI, and API material remain easy to find.
• The docs still build cleanly after the theme and nav changes.