Add docs/index.html landing page; publish.yml substitutes build info

Replaces the inline 4-line index.html written in publish.yml with a
proper text-mostly landing page under docs/index.html — sections for
what it is, install, the loop it replaces, commands, why, and a
'this build' block populated at publish time from VERSION, SHA-256,
and build timestamp via sed.

Single-column, ~44rem max width, system fonts, monospace for code,
dark-mode via prefers-color-scheme. No JS, no images, no remote
assets. ~130 lines of HTML+CSS.