Add searchable API documentation site

[epistemedeus]

Summary

• Add a searchable static documentation site under /docs/.
• Redirect the bare site root to the documentation while preserving links to GitHub and the playground.
• Cover 17 public packages and 197 exported functions, types, methods, variables, and constants from pinned GopherJS commit 490705b.
• Publish generated navigation, search, source-mapped API pages, llms.txt, and a sitemap.
• Keep the reproducible Sourcey 3.6.5 build inputs under sourcey/.

Motivation

This provides a concrete first version of the documentation site requested in gopherjs/gopherjs#1143. It centralizes the public Go API in a project-owned, searchable home while linking users to the existing install guide, compatibility notes, supported-package notes, playground, and pkg.go.dev.

Build

From sourcey/:

npm install
npm run build

The committed godoc.json snapshot means the Pages build does not need a Go toolchain. Generated output is committed so the repository's existing GitHub Pages deployment can serve it without a new CI dependency.

Verification

• Clean npm ci --ignore-scripts completed with zero reported vulnerabilities.
• npm run build produced 19 Sourcey pages and 20 HTML files.
• A static link check traversed 845 local references with zero broken targets.
• Generated API entries link to the exact pinned GopherJS revision.
• No generated file exceeds 1 MB.
• git diff --check passes.

The small postbuild step is documented and deterministic. It works around Sourcey 3.6.5's generated API landing-link mismatch, corrects the local introduction edit link, and normalizes public punctuation.