website: Add GSoC 2026 logo to navbar

[Ishan-AI-coder]

Fixes #4351

What this PR does

Adds the GSoC 2026 logo, label, and "Ongoing" status badge to the top navbar,
using the same pattern previously used for GSoC 2025 (which was already
present in config.toml as a commented-out template).

• Uncommented and updated the [[menu.main]] entry in config.toml to point
  to GSoC 2026 (logo, label, badge).
• Added a stable url front-matter field to
  content/en/events/upcoming-events/gsoc-2026.md, matching the convention
  used in the 2024/2025 GSoC event pages, so the link won't break once this
  page moves to /past-events/.

Testing

Built and previewed locally with hugo server and verified the navbar
renders the logo, label, and badge correctly, linking to /events/gsoc-2026/
without a 404. Also confirmed hugo --gc --minify completes cleanly.

Notes

I'd asked on the issue whether this was still wanted but hadn't gotten a
response yet — opening this PR now so there's concrete code to review.
Happy to close or rework if this turns out to be unnecessary.

[google-oss-prow]

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[google-oss-prow]

Hi @Ishan-AI-coder. Thanks for your PR.

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[Arhell]

/ok-to-test