DOC-3373: Add new .sh script to copy to root post build.

[kemister85]

Ticket: DOC-3373

PR #1: Main Branch (Infrastructure & Build Tools)

Purpose: Add reusable build infrastructure for llms.txt files that will live in /latest version of tinymce-docs ie 8x
Why: Antora doesn't support root-level files by design. Post-build steps are the standard workaround to place files at the site root.

What it does:

• Adds utility script to -scripts/:
  • copy-llms-files.sh — Copies llms.txt files from _attachments/ to site root so they're accessible at /docs/llms.txt
• Updates .github/workflows/preview_create.yml:
  • Adds post-build step to copy llms.txt files to root

Pre-checks:

• Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/.

Review:

• Documentation Team Lead has reviewed