Update README

[IanDelMar]

Description

This pull request removes references to the official theme demo and premium child themes.

Motivation and Context

Both the official theme demo and premium child themes no longer exist, at least not at the indicated location.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (a code change that neither fixes a bug nor adds a feature)
• Styling (changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc))
• Development (changes that affect the build system or external dependencies)
• Documentation (changes that affect existing inline documentation)

Checklist:

• I pulled my branch from develop.
• I am submitting my pull request to develop.
• I have resolved any conflicts merging this pull request would create.
• I have checked that there aren't other open Pull Requests for the same update/change.
• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• (Optional) My change requires a change to the documentation.
• (Optional) I have updated the documentation accordingly.
• (Optional) My change requires a change to the translations.
• (Optional) I have updated the translations accordingly.
• composer phpcs has passed locally.
• composer php-lint has passed locally.
• composer phpmd has passed locally.
• composer phpstan has passed locally.
• I have read the CONTRIBUTING document.

Related Issues or Roadmap requests

Further comments