docs: document custom Mermaid theme override in docusaurus README

## 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` (#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](https://github.com/microsoft/hve-core/actions/runs/35353803755) · auto · 72 AIC · ⌖ 1.75 AIC · ⊞ 9.5K · [◷](https://github.com/search?q=repo%3Amicrosoft%2Fhve-core+is%3Aissue+%22gh-aw-workflow-call-id%3A+microsoft%2Fhve-core%2Fdoc-update-check%22&type=issues)

Read docs/docusaurus/README.md, especially the Accessibility conformance harness section, alongside docs/docusaurus/src/theme/Mermaid/index.tsx, styles.module.css, and __tests__/Mermaid.test.tsx. Update the README to describe the override’s accessibility behavior and explain how its Jest coverage fits the component assertions layer.

Written by the indexing model from the issue text.