Fix documentation and tooling drift

[vndee]

Context

The docs and README still show pip install -e '.[dev]', but development dependencies are defined as a uv dependency group rather than a project extra. mkdocs build -s also reports documentation drift: custom-images.md exists but is not in nav, and several container-pooling links point to missing anchors.

Why this matters

Contributor setup instructions should work from a clean checkout, and docs warnings make it easier for broken links to accumulate unnoticed.

Acceptance criteria

• Update README and docs development setup instructions to use the supported uv workflow.
• Add custom-images.md to the MkDocs nav or remove/merge it intentionally.
• Fix stale container-pooling.md API/configuration anchors.
• Consider making docs link warnings fail CI if supported by the docs tooling.
• Verify mkdocs build -s reports no unexpected documentation drift.