fix: sync tutorial screenshots, resolve indefinite article grammar, ensure consistent indentation for alerts

[MaximilianWenzel]

Handles three minor issues:

• Documentation sync: Replaced 4 tutorial screenshots that displayed the deprecated, parked mysparql domain. They now correctly reflect the safe http://example.org/northwind/ namespace to ensure visual consistency and eliminate any risk of routing users to an untrusted domain.
• Template grammar: Fixed a bug in the templating engine that hardcoded the indefinite article (a {{ relation }}). The logic now correctly evaluates to "an" when appropriate (e.g., "an object property" rather than "a object property").
• Whitespace cleanup: Removed unnecessary 4-space indentations on empty lines within alert boxes.

[rpietzsch]

thank you for the PR. looks good to me.