docs: refresh static site design

[victorquinn]

Summary

• rework with a bright palette, stronger typography, and a new "Under the hood" section that explains the heavy lifting Weather Plus does
• highlight unified schema, high-hit geohash caching, observability telemetry, and provider fallback behaviour
• add Texture branding (wordmark inline in hero + footer) and CTA styling; cap container width for better legibility
• add and link to the docs from the README so Pages can serve weather-plus.dev

Testing

• yarn lint