Add connect-shopify integration documentation

[anyelopetit]

Add Connect Shopify integration guide (EN + ES)

Closes the Shopify item from #5.

What this adds

A complete, bilingual customer-facing guide for connecting a Shopify store to Hellotext, following the exact structure of the existing integration guides connect-wix, connect-woo`).

File	Purpose
_integrations/connect-shopify.md	Collection stub: front matter (title/description/permalink for EN + ES), layout: guide, topic: integrations, popular: false, and the {% translate_file %} include.
_i18n/en/integrations/connect-shopify.md	English guide content.
_i18n/es/integrations/connect-shopify.md	Spanish guide content.
images/integrations/shopify/{en,es}/.gitkeep	Asset directories matching the per-platform image convention, ready for screenshots.
.cspell/custom-dictionary-workspace.txt	Adds Shopify to the workspace dictionary.

Content

• Intro + list of the 6 activities tracked automatically (products viewed,
  abandoned carts, cart updates, completed/updated/refunded orders).
• Step-by-step connect flow: Settings → Integrations → install via the Shopify
  App Store → authorize → optional customer import → confirm.
• "Next steps" section, plus a cross-link to the existing Subscribe on Shopify
  Checkout capture guide (/shopify-checkout / /es/shopify-checkout).

Screenshots

Each step has a placeholder, not a broken image:

<!-- screenshot: images/integrations/shopify/en/integrations.jpeg -->
<p>[screenshot pending: Settings → Integrations]</p>
The HTML comment records the exact intended asset path; the visible caption tells
reviewers what the screenshot should show. The team can drop real images into
images/integrations/shopify/{en,es}/ and swap the placeholders later — no need
to block this PR on screenshots.

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