Give the README a banner that matches goa.design

[raphael]

Replace the README's white, single-layout banner with artwork that matches goa.design: Manrope typography, a navy/blue palette, the two-line headline, and the original round Goa badge. The desktop image shows one contract generating HTTP, gRPC, and JSON-RPC; the mobile layout keeps the headline and transport names readable.

The README uses GitHub's #gh-light-mode-only and #gh-dark-mode-only link markers to select the theme. Each link contains a picture element that selects a mobile PNG by viewport width, falling back to that theme's desktop PNG. Theme selection and width queries are separate because GitHub's themed-picture component replaces theme media queries and discards their width conditions.

The image's alternative text describes the headline and transport scope. Only the first banner block changes; the rest of the README, including the release badge, examples, sponsors, and links, is byte-for-byte unchanged.

The artwork is authored in goadesign/goa.design's scripts/readme-banner.html and regenerated with make readme-banner; all four PNGs are copied unchanged from that source. No application code, dependencies, or public APIs change. No migration or coordinated deployment is needed; reverting this commit restores the previous banner.

Validation:

• GitHub's Markdown renderer retains both theme markers and the mobile picture sources.
• Rendering with GitHub's styles and its actual themed-picture component selects exactly one correct image across all 12 combinations of light/dark/automatic GitHub mode, light/dark system theme, and desktop/phone width, without article overflow.
• All four compositions were visually reviewed. The PNGs are 2400 × 880 for desktop and 1440 × 920 for mobile, at roughly 90–111 KiB each.
• Verified identical image hashes in the website and Goa repositories, and verified that everything after the README's first paragraph is unchanged.