fix: correct typos and grammar errors across pages

[magnus-madsen]

Fixes typos and grammar errors across the site's pages:

index.astro

• "allow" → "allows" (effect-oriented programming)
• "internally uses" → "internally use" (pure functions)
• "expressions holes" → "expression holes"
• "of Flix compiler" → "of the Flix compiler"
• "JVMs exists" → "JVMs exist"
• "thru" → "through"
• Drop redundant "more than" before "4,000+"
• "Tubingen" → "Tübingen" (text and logo alt)
• "Copenhagen University" → "University of Copenhagen"
• Fix sentence fragment: "light-weight threads were added in Java 21"
• Fix stray space in <code> Coll</code>
• Rephrase lattice-semantics sentence for subject-verb agreement

faq.astro

• "recommend to read" → "recommend reading"
• "stop to work" → "stop working"
• "corporations have ever been" → "corporations has ever been"
• "MatLab" → "MATLAB"

contribute.astro

• "interested working" → "interested in working"

get-started.astro

• "recommend to use" → "recommend using"

internships.astro

• "a internship" → "an internship"
• "programme" → "program" (consistency with other pages)

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