Add sponsorship contact CTA to sponsor section

[devin-ai-integration]

Summary

Adds a call-to-action below all sponsorship tiers on the homepage sponsor section: the text "Interested in sponsoring BMT?" and a "Contact Us" button linking to mailto:team@berkeley.mt.

The button reuses the existing button_link shortcode styling (bg-teal-500 etc.) for visual consistency.

In templates/index.html, after the {% for tier in section.extra.sponsor_tiers %} loop:

<div class="flex flex-col items-center gap-3 mt-8">
  <p>Interested in sponsoring BMT?</p>
  <a href="mailto:team@berkeley.mt" class="...teal button styles...">Contact Us</a>
</div>

Link to Devin session: https://app.devin.ai/sessions/2c62d21ae9ab454ca4fe0b9e0d559825

[devin-ai-integration]

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