docs: Remove Google Summer of Code 2025 pages

[alamb]

Which issue does this PR close?

• Part of [Epic] Better / Improved Documentation, Tutorials and Examples #7013
• related to Project Ideas for GSoC 2025 (Google Summer of Code) #14478

Rationale for this change

Now that Google Summer of Code 2025 is done we should clean up the web page

https://datafusion.apache.org/

What changes are included in this PR?

Remove the Google Summer of Code references from https://datafusion.apache.org/

Are these changes tested?

Are there any user-facing changes?

[Jefffrey]

Are these useful enough to be worth archiving on the website itself, instead of removing the pages? I know we'll still have it in git but it might not be as findable in the git history compared to being on the website (but explicitly marked as inactive/old/archived) 🤔

[findepi]

No use to keep them as a statement of record that we had this program and to inspire us next year?

[alamb]

Are these useful enough to be worth archiving on the website itself, instead of removing the pages? I know we'll still have it in git but it might not be as findable in the git history compared to being on the website (but explicitly marked as inactive/old/archived) 🤔

No use to keep them as a statement of record that we had this program and to inspire us next year?

This is a good point -- I could make a "Google Summer of Code" section and maybe leave these pages as "GSOC 2025"