feat: Add Stanford and UC Berkeley AI courses

[sairamsharan]

What does this PR do?

[x] Add resource(s) | [ ] Remove resource(s) | [ ] Add info | [ ] Improve repo

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Description

Adds two foundational, university-level courses to the Deep Learning section: Stanford's CS234 (Reinforcement Learning) and UC Berkeley's CS 188 (Introduction to AI).

Why is this valuable (or not)?

These are highly-regarded, classic courses from top universities that provide a strong foundation for AI and Machine Learning. They are excellent additions to the list.

How do we know it's really free?

Both courses make their lecture notes, videos, and materials publicly and freely available on their official university websites.

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

These are university courses.

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.

Follow-up

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

[github-actions]

Linter failed, fix the error(s):

free-programming-books/courses/free-courses-en.md
652:3-652:89  warning  Remove trailing slash (http://ai.berkeley.edu)  no-url-trailing-slash  remark-lint

[github-actions]

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