[Docs]: Fix API Reference rendering — add model docstrings and tune mkdocstrings config

[Vraj1234]

Location of the issue

• docs/api/datamodels.md — rendered from src/ca_biositing/datamodels/ca_biositing/datamodels/models/** (71 model files)
• docs/api/pipeline.md — rendered from src/ca_biositing/pipeline/
• mkdocs.yml lines 19–32 (mkdocstrings handler config)
• docs/stylesheets/api-reference.css

Suggested fix or improvement

The API Reference pages for Datamodels and Pipeline render poorly — walls of nested headings with no explanatory text. The root cause is two compounding problems:

---

1. Missing class-level docstrings (primary cause)

71 model files across the datamodels package have no class-level docstrings. Every class is just a field list, for example:

```python
class EtlRun(SQLModel, table=True):
tablename = "etl_run"
id: Optional[int] = Field(default=None, primary_key=True)
run_id: Optional[str] = Field(default=None)
started_at: Optional[datetime] = Field(default=None)
...
```

When mkdocstrings renders this, it produces a heading with the class name and then a list of field headings, with zero prose between them. Multiply this across 50+ classes in 14 domain sections and the page becomes an unreadable sequence of headings.

Fix: Add a one-sentence docstring to each class explaining what it represents and when you'd use it. Example:

```python
class EtlRun(SQLModel, table=True):
"""Records a single execution of the ETL pipeline, including status, timing, and record counts."""
```

This is ~71 files, but most classes are simple — estimated 3–4 hours for someone with domain knowledge of the schema.

---

2. mkdocstrings config amplifies the problem

Three config issues make the sparse output worse:

• heading_level: 2 in mkdocs.yml — classes render as ## (h2), their fields as #### (h4). With no prose between the levels, the nesting looks broken and is hard to scan.
• show_if_no_docstring: true in docs/api/datamodels.md — forces every undocumented class to render, producing long pages of bare headings even when there is nothing useful to show.
• Aggressive CSS in docs/stylesheets/api-reference.css — compresses margins and line-height to near-zero, making the already-dense output even harder to read.

Quick config fixes (~1 hour, no docstring work needed):

• Change heading_level from 2 to 3 in mkdocs.yml to reduce nesting depth
• Change show_if_no_docstring from true to false in both API markdown files so undocumented classes are hidden until they get docstrings
• Loosen the spacing in api-reference.css — the current compression was added to work around the nesting problem but makes it worse

---

Recommended approach

Do the config fixes first (fast, immediate improvement), then tackle docstrings incrementally — even partial coverage is better than none, and show_if_no_docstring: false means undocumented classes simply won't appear until they're ready.