docs: refactor and improve rich text docs

[GermanJablo]

• Refactoring that simplifies finding things:

  ## BEFORE
  
  - Rich Text
    - Overview
    - Slate
    - Lexical
  - Lexical
    - Overview
    - Converters
    - Migration
    - Custom Features
  
  ## AFTER
  
  - Rich Text
    - Overview
    - Converters
    - Custom Features
    - Migration
    - Slate (legacy) 

• It takes some of the spotlight away from Slate. Lexical is assumed as the default editor and a banner at the beginning refers to the Slate documentation.

• Various writing improvements.

PENDING:

• some 301 redirects needed
  • /docs/rich-text/lexical to /docs/rich-text/overview
  • /docs/lexical/overview to /docs/rich-text/overview
  • /docs/lexical/converters to /docs/rich-text/converters
  • /docs/lexical/migration to /docs/rich-text/migration

[github-actions]

🚀 This is included in version v3.7.0