docs: route all web-bundles install traffic to bmadcode.com/web-bundles (#2425)

Make bmadcode.com/web-bundles/ the single supported install path. The
site keeps install steps current as Gemini and ChatGPT evolve, always
points at the newest tagged release, and turns one signup into the
notification channel for new bundles.

- README.md: drop direct-folder install reference, point to the site
- web-bundles/README.md: lead with Install (site), reframe folder as
  source for the shelf rather than install target
- docs/explanation/web-bundles.md: replace the per-bundle INSTRUCTIONS.md
  steer with the site, replace the dead how-to link
- docs/how-to/use-web-bundles.md: rewrite as a short pointer to the site
  (kept the file so existing inbound links resolve), retain prerequisites,
  persona customization, and build-your-own sections