docs: render docstring examples as code in the API reference (#1128)

[jepegit]

Closes #1128

Root cause

griffe's Google-style parser (griffelib 2.1.0, _section_kind) only recognises the plural Examples: section title. Eleven docstrings in src/cellpy used Example:, which falls through to a generic admonition whose body mkdocstrings renders as markdown. A line starting with >>> is three nested markdown blockquotes — the three vertical bars in the issue screenshot — and the code becomes plain paragraph text with the prompts eaten. A local zensical build reproduced it exactly (<details class="example"><blockquote><blockquote><blockquote>…).

The same screenshot shows a second format bug: the list_templates Returns: description wrapped at the same indent as its first line, so griffe split it into four dict items.

Fix (docstring level, as the issue preferred)

• Rename all 11 Example: → Examples: (cli_api.py, ica.py ×3, collect/ica.py, collect/dva.py, readers/cellreader.py ×2, parameters/internal_settings.py, batch/facade.py, readers/instruments/base.py).
• The two prose examples (Batch.load, BaseLoader.get_raw_units) switch from RST :: literal blocks to fenced ```python so they highlight under Examples:.
• list_templates Returns: continuation lines indented → one item.
• New tests/test_docstring_sections.py (essential, stdlib ast, <1 s): fails with file:line on any bare Example: title in src/cellpy. On master it lists all 11 offenders; here it passes. Registry row added.
• No theme / template / CSS override.

Before / after

Before (from the issue):

Before: nested blockquotes, plain text, four dict return items

After (rebuilt locally, site/api/cellpy/):

After: single Returns item and highlighted pycon Examples block

How to test

• uv run pytest tests/test_docstring_sections.py — passes; git stash -- src and rerun to see it fail on the 11 old titles.
• uv run --group docs zensical build --clean → No issues found; rg -c '<details class="example"' site/api → no hits (was 4 API pages).
• uv run .issueflows/00-tools/check_docs_relative_links.py → all resolve.
• MPLBACKEND=Agg uv run pytest -m essential → 982 passed, 74 skipped; the 2 test_filefinder.py::test_find_by_project_* failures are the same VM-only ones that also fail on a stashed origin/master here while master CI is green.

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