Update Sage resources in free-programming-books-langs.md

[sstegmanbsd]

What does this PR do?

Added resource

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Description

Added Sage resource

Why is this valuable (or not)?

It is a beginner comprehensive tutorial

How do we know it's really free?

Link is online

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

Checklist:

• 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.

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