Add translation for polish language for all `.md` files

[Marcin Różewski]

English language files remain in the same files, and we are adding new files with Polish translations of the original files, for example:

• The file README.md is the original in English.
• The file README.polish.md is the translation.
• The file how-to-create-kubernetes-cluster.md is the original in English.
• The file how-to-create-kubernetes-cluster.polish.md is the translation.

Other examples of how file names with translations can look:

• README.pl.md
• README.pl-pl.md
• README.polish.md
• README.en.md
• README.en-us.md
• README.english.md