docs: add a five-frameworks image and generate it from the examples

[FabianClemenz]

Summary

The README and the docs home page opened with examples/erd.png — the bare User/Order ERD — under a caption reading "The same ERD from all five frameworks". The picture never showed the five frameworks; the claim lived entirely in the caption. This is the image the launch posts need, and it didn't exist.

Replaced with a composite: the same schema written in SQLModel, SQLAlchemy 2.0, Django, Pydantic and dataclasses, five lines converging on the one diagram they all produce.

Two things keep it honest rather than decorative

• The snippets are trimmed from the runnable examples in docs/examples/* — the same files the docs and the integration tests already use. Real code, not a plausible-looking mock-up.
• The entity boxes use the exact colours generator.py emits — #b8861b for primary keys, the grey for foreign keys, from its !define block. The illustration matches real erdify output instead of approximating it.

No generator script. I initially shipped one and removed it on review, correctly: unlike scripts/gen_cli_docs.py, nothing would enforce that its output matches the committed SVG. The first time anyone nudges the SVG in a vector editor, the script silently regenerates over their edit — a generator you can't trust is worse than none. frameworks.svg is the editable source and carries a header comment saying so, plus the one-liner to re-export the PNG (CairoSVG already ships with the docs group).

Where it's used

File	Change
README.md	Illustrates One Schema, Five Frameworks, above the injected Mermaid ERD — absolute raw URL, since relative paths break on the PyPI rendering
docs/index.md	Home page hero
docs/frameworks/index.md	Top of the overview
docs/examples/erd.png	Removed — unreferenced after the above

Both PNG and SVG are committed: the SVG is the editable artefact, the PNG is what social cards and PyPI can actually render.

Type of change

• Documentation

Verification

mkdocs build --strict passes; pytest (341), ruff check / format, mypy src/ and the full pre-commit run --all-files are clean. The image is 1600×840 (1.91:1, the Open Graph ratio), 71 KB.

I rendered and inspected it, then fixed three things that first pass showed: SVG was collapsing the leading whitespace in every snippet (needs xml:space="preserve", otherwise all five code blocks flatten to one indent level), the converging lines bulged into a lens shape instead of meeting cleanly, and there was dead space under the diagram.

Not done here

The GitHub repo's social preview image is a repository setting, not a file — worth pointing at this PNG in Settings → General → Social preview so links to the repo itself get the same card.

Checklist

• Tests added or updated for the change — n/a, docs asset
• uv run pytest passes locally
• uv run ruff check src/ tests/ and uv run mypy src/ pass
• Docs/README updated if behavior changed
• CHANGELOG.md updated under [Unreleased]

[datadog-devsuit]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 93.29% (+0.00%)

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