Clarify README navigation and distribution

[lilabrooks]

What changed

• adds quick links for the live showcase, latest release, consumer ZIP, and W3C validation
• adds a compact table of contents
• explains the difference between the clean distribution ZIP and GitHub source archives
• adds a browser, viewport, visual, accessibility, fallback, HTML, packaging, and tool-consistency matrix
• consolidates installation, theme behavior, layout, Pages, release, and licence copy

Length review

The revised README is 808 words, down from 867 words before this update.

Validation

• npm run test:static
• npm run test:pages
• public artifact remains exactly 11 allowlisted files