docs: landing redesign, design system, and page consolidation

[vboussot]

Summary

Docs-only overhaul aimed at adoption: a redesigned landing, a site-wide design system, and a consolidation pass that removes duplicated pages so every topic has exactly one canonical home.

Landing & design system (5f28219)

• New landing (index.rst): hero pairing a real Config.yml with the three CLI commands; a file → command → outputs mental-model table with the root keys visible (Trainer: / Predictor: / Evaluator:); three pillar cards; intent-based next-step cards; a full documentation index; MICCAI proof strip.
• Design system (_static/custom.css): two-theme token palette sampled from the logo ink — teal as the brand voice, steel blue as the machine voice (code surfaces only), violet/coral/amber as category hues. Dark mode follows Shibuya's html.dark plus a prefers-color-scheme fallback.
• Global content-page layer: typed admonitions (note/tip/warning/danger each get their hue), tables, code blocks, toctree lists and page leads inherit the design automatically — no per-page HTML.

Consolidation & orientation (55c9e7e)

Eleven redundant pages merged into their canonical homes, with every unique fact preserved:

Removed	Residue folded into
usage/{training,prediction,evaluation}	config_guide/{…} (+ new Running it sections: commands, RESUME, -tb, --evaluations-dir)
usage/remote-server, concepts/apps	usage/apps (bundle anatomy + remote execution)
config_guide/patterns	concepts/datasets
concepts/imaging-formats	reference/components/storage-backends
contributing, architecture, usage/testing	development (CI matrix, repository map, test loops)
reference/api/public-api	reference/api/index

• examples/segmentation, examples/synthesis and usage/docker are now single-sourced as MyST includes of the repository READMEs — no more diverging copies.
• Orientation pass on every remaining page: a purpose lead ("what this page gives you, when you need it"), pruned Next steps (2–3 reasoned links), reading-order guidance on section indexes, and explicit success signals after each quickstart command.
• Apps trust-model wording unified on the behaviour committed on main (requirements install is opt-in, off by default).

Verification

• sphinx-build passes with zero broken-reference or orphan warnings (remaining warnings are the pre-existing autodoc import noise).
• All landing links resolve; include pages render; each merge residue verified present in the built HTML of its surviving page.

Preview: pixi run build-docs then open docs/build/html/index.html.