docs: establish screenshot conventions and add dashboard screenshots across the docs

[IvanHunters]

Part of #605.

The repo already ships layouts/shortcodes/blocks/flux_ui_galleries.html and home/screenshot-gallery.html, but they are used only on the homepage. The docs describe the UI entirely in prose; the only dashboard screenshots in the site are in the 1.4 blog post.

Deliverables:

• Capture a current, consistent set of 1.4 dashboard screenshots (resolution/theme/redaction conventions).
• Reuse the existing gallery shortcodes to embed them in the getting-started tutorial and the new dashboard guide (docs: add a "Using the Dashboard" guide with screenshots #605).
• Document the dual "Cozystack Dashboard / kubectl" tab pattern (already used in getting-started/deploy-app.md) as a content convention in CONTRIBUTING.md.