[lexical-website] Documentation Update: Consolidate scattered React docs

[anuj-kuralkar]

Current behavior: React integration docs are scattered across getting-started/, collaboration/, and extensions/, mixing with Vanilla JS concepts.

Changes added by this PR:

• Moved files: Relocated getting-started/react.md, collaboration/react.md, and extensions/react.md into docs/react/.
• Split Migration Guide: Separated docs/extensions/migration.mdx into React (docs/react/migration.mdx) and Vanilla (docs/extensions/migration.mdx) versions.
• Renamed Headers & Titles: Removed redundant "React" wording from <h1> tags (e.g., # Getting Started with React -> # Getting Started) and updated the React migration guide title to # Migrating to Extensions.
• Sidebar: Grouped all React docs under a single "React" category in sidebars.js.
• Validation: Fixed internal relative markdown links so Docusaurus builds cleanly.

Note: This is a structural file reorganization. Core content was not rewritten.

Before

docs/
├── getting-started/
│   └── react.md                  <-- React clutter
│
├── extensions/
│   ├── migration.mdx             <-- Mixes Vanilla & React migration
│   └── react.md                  <-- React clutter
│
├── react/                        <-- Incomplete React folder
│   ├── index.md
│   ├── plugins.md                
│   ├── create_plugin.md
│   └── faq.md
│
└── collaboration/
    └── react.md                  <-- React clutter

After

docs/
├── getting-started/              (React clutter removed)
│
├── extensions/                   
│   └── migration.mdx             (React clutter removed)
│
├── react/                        <-- ALL React integration now lives here
│   ├── index.md                  
│   ├── getting-started.md        (Moved from getting-started/react.md)
│   ├── plugins.md                
│   ├── create_plugin.md          
│   ├── collaboration.md          (Moved from collaboration/react.md)
│   ├── extensions.md             (Moved from extensions/react.md)
│   ├── migration.mdx             (Split from extensions/migration.mdx)
│   └── faq.md
│
└── collaboration/                (React clutter removed)

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Project	Deployment	Actions	Updated (UTC)
lexical	Ready	Preview	Jul 18, 2026 7:17pm
lexical-playground	Ready	Preview	Jul 18, 2026 7:17pm

[etrepum]

I don't think this necessarily makes sense, a reorganization could be done in the context of a larger documentation update but there's not a lot of reason to just move things around.