Summary

Implements the screenshot workflow described in #3081.

Adds a docs/screenshots.py module that renders Click help output as
annotated terminal-style PNG images using Pillow. The renderer:

• Runs locally (no external API calls)
• Works in CI (all deps are pip-installable, no browser binary needed)
• Produces crisp images at 2x scale for high-DPI displays
• Draws colored annotation boxes with labels around CLI sections
• Auto-detects system monospace fonts on Linux/macOS/Windows

Screenshot

Changes

• docs/screenshots.py — New screenshot generator module
• pyproject.toml — Add pillow to docs dependency group
• docs/_static/cli-help-annotated.png — Generated screenshot
• docs/quickstart.md — Add "Understanding the Help Output" section

Design Decisions

• Pillow over headless browser: lighter weight, no browser binary
  needed in CI, fully pip-installable.
• Auto-detect system fonts: avoids bundling font files; falls back
  to Pillow's built-in font if none found.
• Annotation by text matching: annotations use start_text prefix
  matching to find line ranges, making them robust to output changes.
• Standalone script: can be run manually (python docs/screenshots.py)
  rather than integrated as a Sphinx extension, keeping the build simple.

Closes #3081.