Add authors, fix dead links in free-courses-pl.md

[MixeroTN]

This PR adds authors, removes one dead link and replaces one dead link with the last version from web archive (with 'archived' notation) in the free-courses-pl.md file.

The "Bash" link to YouTube playlist was also invalid but I found the author and recreated the playlist by myself 😉

Following links have been removed:

• Kurs JavaScript - Jakub Jurkian (invalid link, can't find the author)

I have not changed anything in any file except the one i have worked in.

Why is this valuable (or not)?

This is valuable because it adds the authors of the courses and ensures that all links work.

Checklist:

• Read our contributing guidelines.
• Search for duplicates.
• Include author(s) and platform where appropriate.
• Put lists in alphabetical order, correct spacing.
• Add needed indications (PDF, access notes, under construction).
• Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!