docs: make website README clearer for public visitors

[eirabie]

Summary

Public visitors need a clear introduction and setup links before development details. The README now leads with OpenAthan's purpose, installation and recovery links, supported hardware, and the fresh-install data-erasure notice. It keeps existing-owner update guidance prominent, describes release checks as scheduled twice hourly, and moves simulator, development-server, and redirect details into contributor guidance.

Area

• Homepage or public documentation
• Hardware/build guidance
• Browser installer
• Project or deployment configuration

Validation

• Relevant routes and links were checked
• User-facing behavior was tested in applicable browsers and viewport sizes — not applicable to documentation-only changes
• Accessibility impact was reviewed
• No firmware is compiled by this repository

Formatting, whitespace, and link checks passed, and the rendered README was visually reviewed in Chromium. Website CI passed on commit e67e538. Physical installer validation is not required for this documentation-only change.

Licensing and privacy

• No new third-party media or dependency licensing concerns
• No new collection or transmission of user data
• Any licensing, attribution, or privacy impact is documented — existing licensing and location-service disclosure are retained