feat(landing): redesign homepage with product-hunt-ready marketing layout

Replace the generic feature-grid hero with a conversion-oriented landing
page that leads with "Replace 5 databases with 1 universal engine". New
layout includes a before/after stack comparison, a GraphRAG code window,
a waitlist form backed by MailerLite, and a Product Hunt banner.

Switch brand color from blue (#3B82F6) to indigo (#6366F1) across CSS
variables and oxidoc.toml to align with the nodedb.dev marketing site.
The landing page is dark-only (scoped to .oxidoc-landing) while docs
continue to respect the user's system preference.

Register the five custom JS components in oxidoc.toml and add the
Use Cases nav link and section routing.

Author: farhan-syah