feat: fall 2025 challenge

[BekahHW]

Linked Issue

Description

This pull request updates the monthly challenges page to highlight the current challenge and updates the "Preptember" challenge. It also removes outdated documentation about managing newsletters from the README.md. The key changes are as follows:

Monthly Challenges Page Updates

• Marked "Preptember" as the current challenge by setting current: true and removed the current: true flag from the "Photography Challenge" to ensure only one challenge is marked as current at a time. [1] [2]
• Enhanced the "Preptember" challenge's subtitle and description to better explain its purpose and structure, emphasizing skill-building and preparation for Hacktoberfest.
• Updated the link in the "Preptember" challenge details to point to the "Fall 2025" challenge page, and added a new link for "September, 2024" to the list of related links.

Documentation Cleanup and Update

• Removed outdated instructions from the README.md regarding how to add and index newsletters, as this process is no longer relevant.
• Added link to "Monthly Challenge Technical Guidelines" section for updating monthly challenge page.

Methodology

Code of Conduct

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[adiati98]

Thanks so much, @BekahHW! 🚀