Convert docs to single-page tabs and update styles

Merge API and About pages into a single SPA-style index: removed docs/about.html and docs/api.html and moved their content into docs/index.html as tabbed sections (start, api, docs, notfound). Add client-side routing and history handling via a new script to support deep links and pushState, plus a Netlify-style docs/_redirects rule to serve index for all routes. Update styles in docs/main.css to target the new tab UI (.tab, .tablinks, .logo-text) and add styles for the 404 view. Minor markup and layout tweaks (images, buttons, response examples) to match the consolidated page.

docs/_redirects

@@ -0,0 +1 @@
+/* /index.html 200