feat(core,docs): add AccessibleImageBlock (alt=) and wire into homepa…

[Ovgodd]

Title

Add AccessibleImageBlock for decorative images (alt="", role="presentation", aria-hidden, tabindex)

Why
Avoiding screen reader to read alt,
RGAA 1.2 criterion

Description

This PR introduces a reusable opt‑in image block, AccessibleImageBlock, exported from @blocknote/core. It renders images as decorative by default (alt="") and sets role="presentation", aria-hidden="true", and tabindex="-1". The homepage demo is updated to use this block via a schema override.

Details

• Added AccessibleImageBlock to core:
• File: packages/core/src/blocks/ImageBlockContent/AccessibleImageBlock.ts
• Exported via packages/core/src/index.ts
• Implementation wraps imageBlockConfig with a custom render that:
• Sets alt="" (decorative image per WCAG 1.1.1)
• Sets role="presentation" and aria-hidden="true" so assistive tech ignores the image
• Sets tabindex="-1" so the image isn’t focusable
• Reuses existing imageParse and imageToExternalHTML

Demo integration:
docs/app/(home)/hero/DemoEditor.tsx overrides the schema to use AccessibleImageBlock for the image block type

Usage :

Override the image block in your schema:

import { BlockNoteSchema, defaultBlockSpecs, AccessibleImageBlock } from '@blocknote/core';

const schema = BlockNoteSchema.create({
  blockSpecs: {
    ...defaultBlockSpecs,
    image: AccessibleImageBlock, // alt="" by default
  },
});

React:

import { useCreateBlockNote } from '@blocknote/react';
const editor = useCreateBlockNote({ schema });

Vanilla:
React:

import { useCreateBlockNote } from '@blocknote/core';
const editor = useCreateBlockNote({ schema });

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