[github-actions]

Bumps the version from 0.3.0 to
0.4.0 in pyproject.toml and doc/source/conf.py.

Changes in this release

• Docs: identical top bar on every page, full width, bigger fonts, simpler
• Feature: redesign the docs site as a widget-style app
• Settings widget: Browse to the right of the field, no "upload" prompt
• Settings widget: client-side folder Browse
• Feature: an editable settings widget
• Docs: document the settings model and shared data
• Feature: matplotlib-style settings + system-wide data resolution

To publish

1. Merge this PR.
2. Draft a release
   tagged v0.4.0 against main.
3. Publishing it triggers publish.yml, which checks the tag against
   pyproject.toml and uploads to PyPI.

CI does not start on this PR by itself - it was opened by a workflow. Close
and reopen it if you want build.yml to run before merging.

Opened by the prepare release workflow.