docs: host nexus narrative docs the kameo way (docs.page)

[joeldsouzax]

Adopt kameo's documentation hosting approach for nexus.

The kameo way

kameo hosts its narrative docs on docs.page (Invertase) at https://docs.page/tqwewe/kameo — zero-build, renders .mdx + a docs.json config straight from the GitHub repo (no CI, no static-site build step, free, custom-domain capable). API docs are separate on docs.rs.

docs.json drives it: name, description, logo.light/dark, socialPreview, optional googleAnalytics, and a sidebar of groups → pages (title/href/icon).

Tasks

• Add a docs/ tree of .mdx pages for nexus (CQRS/ES/DDD/hexagonal concepts, the nexus integration contract, store/kernel guides, examples).
• Add a docs.json (nexus branding, sidebar, own googleAnalytics if wanted).
• Go live at https://docs.page/devrandom-labs/nexus (zero-build); optional custom domain.
• Keep API docs on docs.rs/nexus (separate track).

Why

• Zero-build → nothing added to nix flake check; docs ship straight from the repo.
• Same toolchain as kameo/bombay → one docs story across the devrandom stack.

Refs

• Model: https://docs.page/tqwewe/kameo (config: kameo docs.json + docs/*.mdx).
• Sibling card: bombay does the same (vendors kameo's docs.page site).