Add donate section to homepage with impact blurb

[devin-ai-integration]

Summary

Adds a "Support BMT" section on the homepage, placed above the sponsors, with a left-aligned impact blurb and a Donate button linking to a new Stripe Payment Link (https://donate.stripe.com/5kQeVf9mlcCT4Gx6eX9Zm0O).

The blurb emphasizes that BMT is run entirely by 100+ UC Berkeley student volunteers and cites this season's reach (5,000+ students from 600+ schools) — numbers pulled from ContestDojo (BMT 2025, BMT 2025 Online, BmMT 2026, BmMT 2026 Online: 5,133 students on teams across 625 distinct orgs; 100 distinct staff names on the about page). Also bumps the intro's "over 4,000 annual student participants" to 5,000.

The Stripe side was set up via API on the live BMT account: a "Donation to Berkeley Math Tournament" product with a customer-chooses-amount price (preset $25, min $1, USD) and a payment link with submit_type=donate.

Link to Devin session: https://app.devin.ai/sessions/15c4f3ee41584b82ab43196e91e83768
Requested by: @oliver-ni

[devin-ai-integration]

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[devin-ai-integration]

Tested locally with zola serve — the donate section and Stripe checkout both work end-to-end.

Donate section (above sponsors, left-aligned)	Stripe checkout ($25 preset)

• "Support BMT" section renders above the 2025–2026 Sponsors section, left-aligned, with the volunteer/impact blurb and Donate button
• Intro paragraph updated to "over 5,000 annual student participants"
• Clicking Donate opens the live Stripe checkout: "Donation to Berkeley Math Tournament", editable amount preset to $25.00
• No payment was submitted (live link)

Editable amount verification