docs: rewrite all docs for the power-markets platform + publish to GitHub Pages

[oldhero5]

Redoes the entire documentation site to match the current platform and sets up publishing.

What changed

• All 11 Docusaurus pages rewritten to the current reality (10 via a parallel rewrite pass against a shared ground-truth brief + the live code; architecture.md by hand): power-markets focus (EIA-930 across all ~65–73 balancing authorities + ERCOT real-time/day-ahead SPP and system load), the live S2 read API, the bitemporal ArcticDB model, the operating-day-aware Dagster schedules, and content-idempotent storage. The old docs predated the power pivot and described the S2 API as "reserved" with invented endpoints.
• Docusaurus config: fixed baseUrl to the project-pages path /energex/ and refreshed the tagline.
• Publishing: added .github/workflows/docs.yml that builds the site and deploys it to GitHub Pages (source = GitHub Actions) on every push to main that touches website/**. Pages is enabled; the site will live at https://oldhero5.github.io/energex/.
• Repo About + homepage updated separately (the old description still referenced the removed DuckDB futures library).

The site builds clean (npm run build, strict broken-link check passes).