Add Polish language setup and Hello Nextflow intro translation

[itrujnara]

• Add Polish to mkdocs.yml
• Add Polish glossary to TRANSLATION.md
• Add translation of index.md in Hello Nextflow

Do not merge this yet, as I need to find Polish-speaking reviewer(s) to check the linguistic side of this

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[mribeirodantas]

Thanks for starting this, @itrujnara! I messaged a couple polish Nextflow enthisiasts to check if they're available to help with / review the polish translation of the Hello Nextflow training 😉.

[mribeirodantas]

Hi @itrujnara. Do you plan to cotinue translating this training piece?

[itrujnara]

Hi @mribeirodantas, I am still interested in translating the training materials, but I haven't managed to find any other Polish-speaking Nextflow user to review the PRs, so this will have to stay in draft for some more time, unless you want me to close it instead.