feat: use the search terms people type for a self-hosted shop

[PhilippTheServer]

Closes #4

The title, meta description, H1, lede and "What OpenTaberna is" copy now use the terms people search with (open-source, self-hosted, headless shop, web shop, e-commerce, modern, custom, Shopware alternative). The Organization JSON-LD description and llms.txt match. check_site.rb fails if the title, description or H1 lose the core terms; the README documents that.

New title: OpenTaberna — Open-source, self-hosted headless shop system

Verification

./scripts/verify.sh (Ruby 3.3 container)

before the copy change:
check_site.rb: 7 failure(s)
  ✗ index.html: title has lost the search term "open-source"
  ✗ index.html: title has lost the search term "self-hosted"
  ✗ index.html: title has lost the search term "headless"
  ✗ index.html: title has lost the search term "shop"
  ✗ index.html: meta description has lost the search term "self-hosted"
  ✗ index.html: h1 has lost the search term "open-source"
  ✗ index.html: h1 has lost the search term "headless"

after:
check_site.rb: all checks passed
==> all verification passed

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