Update meetup content for September 2026 Practi"FALL" Prints challenge

- Archive August 2026 Gridfinity Extravaganza
- Set September 2026 Practi"FALL" Prints as active challenge
- Set October 2026 as upcoming challenge
- Create announcements/September2026.md and challenges/September-2026/
- Update Important-Dates, Readme, challenges.md, and sync wiki-content-gitea
- Update wiki-fargo3d submodule pointer

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

announcements/September2026.md

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+# 3D Fuel September Meetup - Meeting Announcement
+
+## Email Subject Line
+
+Join Us in September: 3D Printing Meetup + Practi"FALL" Prints Challenge!
+
+---
+
+## Email Body
+
+Hey everyone!
+
+Join us for the **September 3D Printing Meetup**.
+
+For more information about the meetup and community events, visit [www.fargo3d.com](https://www.fargo3d.com)
+
+**Date: Wednesday, September 16th, 2026**  
+**Time: 6:00 - 8:00 PM**  
+**Location**: West Fargo Public Library, 215 Third St. E., West Fargo, ND 58078
+
+Come join us to catch the latest industry news updates from John and share your practical 3D prints!
+
+### September Challenge: Practi"FALL" Prints
+
+As we head into fall, it is time to show off the practical side of 3D printing! Bring in a functional print that solves a problem, repairs something broken, or makes everyday tasks easier.
+
+**Challenge Requirements:**
+- Bring your **practical/functional print** (anything from a simple door stop, hook, or clip to a complex bottle/can opener, tool jig, or custom bracket).
+- Be ready to share what problem it solves, how you use it, and what slicer/material choices you made (infill, perimeters, material strength)!
+
+Whether you designed it yourself or found a great model on Printables/MakerWorld/Thangs, we want to see how you use 3D printing to fix real-world problems.
+
+See you there!