docs: improve structure and add interactive architecture diagram

Summary

Reorganized the documentation site so pages are grouped by what a reader is trying to do — learn (Tutorials), accomplish a specific task (How-to guides), understand a concept (Concepts), or look something up (Reference) — instead of mixing all of these together under "Tutorials." Also refreshed the homepage and split a couple of overloaded pages into focused ones. This is inspired by the well documented Diátaxis that we almost adhered to anyway and used in other proprietary packages.

Where things moved

• A lot of content that used to live under Tutorials (accessing services, the lab repository, installing packages in editable mode, run IDs, parameter monitoring, alerting, reporting, and all the extension-development guides) has moved to a new How-to guides section. Old links and bookmarks to these still work — they redirect automatically to the new location.
• Two pages that explained background concepts (differences from the legacy Juice system, environment/packaging internals) moved from Reference into Concepts. These also redirect automatically.
• Heads up on two exceptions that don't auto-redirect (because the content moved to a brand-new page rather than an existing one moving):
  • Admin tasks that used to be in "First steps after installation" — adding users, adding services, adding database storage, installing a package for everyone — are now on How-to guides → Administration → System configuration.
  • The "parameter monitoring and throttling" walkthrough that used to be inside the "Running an Experiment" tutorial is now its own tutorial, Parameter Monitoring and Throttling, placed right after Fridge Monitor.
  • If you have either of these bookmarked at their old in-page location, update the link — the general page still loads, but that specific section is no longer there.

New or reworked content

• Running an Experiment now flows into a second tutorial, Scheduling your experiment, instead of covering both in one long page.
• Parameter Monitoring and Throttling is a new, hands-on tutorial (see above).
• The homepage now has an interactive architecture diagram — hover or click a component to see what it does — and the old separate "introduction" page was merged into the homepage itself.
• Tutorials that had "screenshot coming soon" placeholders now have real screenshots.

Not changed: nothing about how Juice itself behaves — this is a documentation-only update.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com