minor fixes to the documentation

[dvirf1]

Fixing minor inconsistencies while I was reading the documentation:

• The given-clauses page refers to "the given instances defined previously". Those instances were defined in camelCase and not PascalCase, so the instances are now consistently called intOrd and listOrd in both pages.
• The Scala version in multiversal-equality should be 2.13
• relationship-implicits: implicitly[A] in Scala 2 was changed from the[A] to summon[A] in Scala 3 + renaming Deco to Decorator to be friendlier for new developers.
• dependent-function-types: fixing the result type of extractKey and describing the function with e

[dottybot]

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[smarter]

Thanks for your contribution!