Clarify that Week 2 Python challenge is optional

[Yosolita1978]

📝 Description

Follow-up to #2754. Updates the Week 2 Code Challenges README to make clear that
the JavaScript challenges are the required track for program participants and
that the Python version is optional. Applies the same feedback Naomi gave on the
Week 3 README (#2756), which pointed out that the curriculum also serves grads,
seeker-program participants, and independent learners.

🔂 Changes Made

• Added a "Who This Folder Is For" section naming the three audiences.
• Retitled "Why You Solve Some Challenges Twice" to "Duplicated Exercises" and
  removed the text encouraging learners to solve the challenge in both languages.
• Listed the three JavaScript challenges as the main list, with
  string_manipulation.py under an "Optional:" heading.
• Removed the learning objective about explaining the idea in both languages.
• Scoped the Python prerequisite, doc link, and index note as optional.
• Kept the connection between Find Last Index and Find Target Index, which is a
  genuine conceptual link rather than a duplicate, and moved it under the vital
  concept.

⚙️ Related Issue

• Related to Add Week 2 Code Challenge clarifications #2627 (already closed by Add Week 2 Code Challenges README (closes #2627) #2754); follows the review feedback on Add Week 3 Code Challenges README (closes #2628) #2756.
• Closes issue Week 2 README: "Why You Solve Some Challenges Twice" section incorrectly implies Python version is required #2759

🍏 Type of Change

• Documentation update

🎁 Acceptance Criteria

• The README makes clear that the JavaScript challenges are the required track
  and the Python version is optional.
• No section asks a program participant to complete a Python exercise.
• Every file referenced in the README matches a file that exists in the folder.

🧪 How to test or what to evaluate

1. Open code-challenges/Week_2/README.md and preview the Markdown (VS Code: Cmd+Shift+V).
2. Confirm all headings render and all links are valid.
3. Cross-check the filenames listed against the folder contents.
4. Confirm no section asks a program participant to complete the Python challenge.

🚀 Repo Notes (if applicable)

• Table of contents: no update needed; this is a folder-level README.
• Other repo areas impacted: none — documentation only.
• Full-time program considerations: program participants work in JavaScript only,
  and the README now reflects that.

📸 Screenshots (if applicable)

N/A — Markdown documentation only.

✅ Checklist

• I have performed a self-review of my code.
• My code follows the style guidelines of this project.
• I have commented my code where necessary.
• I have tested my code locally and verified the website is working as expected.
• (if applicable) I have added documentation in the README.
• (if applicable) I have added tests that prove my fix is effective or that my feature works.
• (if applicable) New and existing unit tests pass locally with my changes.

[naomiquinones]

Thanks for the updates. I noted a few other things that could be clarified.

[naomiquinones]

Lines 28-30 above already indicate that the python file is the same exercise in a different language, so these lines are redundant and can be removed.

[naomiquinones]

Should these paragraphs have a title to separate them from the "Duplicated Exercises" section that precede them? Or maybe they could be moved to the Motivation or Learning Objectives section.

[naomiquinones]

This might be the place to move the paragraphs that go into more detail about the -1 pattern