Update prompt and instructions files

[ntrogh]

Updating documentation for prompt and instruction files:

• Instructions files

  • .instructions.md files
  • User and workspace instructions files
  • Sync with Settings Sync
  • Add applyTo metadata to automatically assign to specific files
  • Commands for creating instructions files

• Prompt files

  • User and workspace prompt files
  • Metadata fields for mode, description, tools
  • Template variables
  • Run from editor with play button
  • Run with /
  • Save prompt with /save
  • Commands for creating and running prompt files