Add AtomVM Ecosystem page

[atsetilam]

Summary

Adds a new /ecosystem/ page that showcases the products, community projects,
and strategic contributors built around AtomVM. The page is linked from both the
main navbar and the footer.

What's included

Three categories, each entry rendered as a card (logo, description, and external
links), styled to match the existing homepage feature cards:

• Commercial Products — La Machine
• Third-Party Projects — midiMESH
• Strategic Ecosystem Contributors — NLnet Foundation (NGI0 Commons Fund), piyopiyo.ex

Layout / UX details

• Categories that contain a single entry are centered rather than left-aligned.
• The NLnet logo swaps between a dark variant (light theme) and a light variant
  (dark theme) so it stays legible in both color schemes.

Files

• ecosystem.md, _layouts/ecosystem.html, _includes/ecosystem_card.html — page, layout, reusable card include
• _data/ecosystem.yml — page content (headings, cards, copy)
• _sass/_ecosystem.scss, assets/css/main.scss — styles
• _data/home.yml — navbar + footer links to the new page
• assets/images/ecosystem/* — logos