Update howto-pl.md

[jarek-pacocha]

What does this PR do?

Improve repo

• Added further translations and some changes to the phrasing.
• Fixed a broken link that pointed to a nonexistent translation and changed it to point to the original EN version untill translation is created.

For resources

Description

Why is this valuable (or not)?

How do we know it's really free?

For book lists, is it a book? For course lists, is it a course? etc.

Checklist:

• Read our contributing guidelines.
• N/ASearch for duplicates.
• N/A Include author(s) and platform where appropriate.
• N/A Put lists in alphabetical order, correct spacing.
• N/A 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!