fix(docs): restore Mermaid theme configuration

[cofin]

Summary

• replace the removed mermaid_init_js setting with the supported mermaid_init_config API
• configure Mermaid's native light and dark themes and update the bundled Mermaid version to 11.12.1
• disable Shibuya's SVG inversion so it does not double-correct Mermaid's native dark theme

Why

sphinxcontrib-mermaid 2.x no longer registers mermaid_init_js, so Sphinx silently ignored SQLSpec's diagram configuration. The extension already renders and re-renders native light and dark themes, while Shibuya's dark-mode filter inverted the resulting dark SVG and produced washed-out diagrams.

Closes #675