Add cross-reference link between GSoC 2025 ideas and classics pages

### Description
The **"Google Summer of Code (GSoC) 2025 ideas page"** and the **"Google Summer of Code (GSoC) 2025 ideas page - classics"** are related but currently lack a clear link between them. This can cause confusion for users who are using for the first time, trying to navigate between the two pages.

### Proposed Solution
Add a **cross-reference link** near the top of each page to help users easily navigate between the main ideas page and the classics page. For example:
- On the **main ideas page**, add:
*"For classic, well-established project ideas, check out the [GSoC 2025 Classic Projects](#) page."*
- On the **classics page**, add:
*"For new and evolving project ideas, check out the [GSoC 2025 Ideas Page](#)."*

This will improve navigation and make it easier for users to find the information they need.