docs: document custom Mermaid theme override in docusaurus README

[github-actions]

Component

Documentation

Bug Description

The push that upgraded Mermaid (commit e37d8d60, "fix(docs): preserve Mermaid accessibility and grader lineage", #2927) added a new custom theme component at docs/docusaurus/src/theme/Mermaid/ (index.tsx, styles.module.css, and __tests__/Mermaid.test.tsx) that serializes render attempts, preserves diagrams during keyboard theme transitions, and surfaces render errors through an alert with deterministic retry focus. docs/docusaurus/README.md was not updated to describe this swizzled component, its accessibility behavior, or how it fits into the existing "Accessibility conformance harness" section (static lint, component assertions, behavioral e2e, full-site crawl).

Expected Behavior

docs/docusaurus/README.md should mention the src/theme/Mermaid override: what it does (render-attempt serialization, diagram preservation across theme changes, retry-focus error alert), why it exists (accessibility requirements for keyboard/theme transitions and error recovery), and how its Jest coverage (__tests__/Mermaid.test.tsx) relates to the documented "Component assertions (jest-axe)" layer.

Steps to Reproduce

1. See commit e37d8d60 (fix(docs): preserve Mermaid accessibility and grader lineage #2927), which added docs/docusaurus/src/theme/Mermaid/index.tsx, styles.module.css, and __tests__/Mermaid.test.tsx.
2. Open docs/docusaurus/README.md and note no section describes this custom theme component.

Additional Context

• New files: docs/docusaurus/src/theme/Mermaid/index.tsx, docs/docusaurus/src/theme/Mermaid/styles.module.css, docs/docusaurus/src/theme/Mermaid/__tests__/Mermaid.test.tsx
• Documentation to update: docs/docusaurus/README.md (Accessibility conformance harness section)

Generated by Documentation Update Check · auto · 72 AIC · ⌖ 1.75 AIC · ⊞ 9.5K · ◷