[ruby/en] Update ruby.md by adding array method

[Wessel-Boers]

Array#append is more idiomatic in modern Ruby (post-2.5) for its clarity and descriptive name, although Array#push is still frequently used due to its long history and prevalence in older codebases.

It would be a good practice for Ruby beginners to use append for new code.

Also, this would more intuitively help them remember Array#prepend for adding to the start of an array (as opposed to Array#unshift)

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