Fix SEO and metadata — OG image, canonical URLs, structured data

[Gui-FernandesBR]

Description

The SEO setup is the strongest aspect of the current site, but there are a few remaining issues:

1. og:image shows poorly on social media — currently points to logo-white.png (white logo on transparent background), which is invisible on white-background platforms (Facebook, LinkedIn, iMessage previews)
2. Copyright year is hardcoded — footer shows "RocketPy Team © 2025", should be updated to 2026 (or made dynamic)
3. Missing canonical link — no <link rel="canonical"> tag to prevent duplicate content issues
4. Missing structured data — no JSON-LD schema markup (SoftwareApplication schema would help search engine rich results)
5. Sitemap references — sitemap.xml should be referenced in robots.txt
6. Image alt text — decorative <div> elements with role="img" could use more descriptive aria-label values, and converting them to real <img> tags would improve SEO

Proposed fixes

• Create a proper OG image (1200×630px) with the RocketPy logo on a dark blue background
• Update copyright year to 2026 (or use JS to auto-update)
• Add <link rel="canonical" href="https://rocketpy.org/"> to both pages
• Add JSON-LD structured data for SoftwareApplication
• Add Sitemap: https://rocketpy.org/sitemap.xml to robots.txt

Affected files

• index.html — canonical link, structured data
• about.html — canonical link
• robots.txt — add sitemap reference
• images/ — new OG image asset
• Footer in both HTML files — copyright year

Acceptance Criteria

• OG image displays correctly on Facebook Debugger and Twitter Card Validator
• Copyright year is current
• Canonical URLs are set on both pages
• robots.txt references sitemap.xml
• (Stretch) JSON-LD structured data present and valid