feat(generator): docs_kit:page + Registry v2 — one-line page registration - #29

Closes #12
Closes #13.

Also lands #12 (Registry v2), which #13 depends on — the generator injects the page "Title", group: "…" line that #12 introduces, so they ship together.

Why

Adding a docs page today is quadruple bookkeeping: an 8-line module Views; module Docs; module Pages nesting ceremony, a class body, AND a registry line whose four strings must match the class — forget either half and you silently get a missing page. This is the moment a developer decides documentation is too much work.

After this PR, adding a page is one command plus writing content:

rails g docs_kit:page "Getting Started" --group=Guide

What changed

Registry v2 (#12)

• DocsKit::Registry gains an opt-in page(title, group:, slug:, view:, icon:) DSL with path_prefix / view_namespace. slug/view derive from the title (parameterize / camelize), both overridable per line. A default Entry gives readers + href + view_class (safe_constantize under view_namespace, nil until authored — no dead links). nav_items returns { group => [NavItem] } for authored pages only — the transform every site used to hand-write. Mixing page and entries raises Registry::Error.
• Configuration#nav_registries (default {}): nav_groups derives the whole sidebar from it (dropping empty groups). An explicit c.nav lambda still wins — full backwards compatibility.
• The gem requires the ActiveSupport inflections core-ext so it derives slugs/views standalone (a host app already has it).

Page generator (#13)

• lib/generators/docs_kit/page/ writes the page in the compact class form (class Views::Docs::Pages::X < DocsUI::Page) — Zeitwerk resolves it through the directory-implied namespaces, killing the nesting tax — with a starter DocsUI::Section + Markdown island.
• Injects page "Title", group: "Group" after the last page line, so ordering lands at the group's end.
• --slug / --view / --eyebrow / --registry override every derivation.
• Idempotent: no duplicate registry line, no clobbered file in --skip mode.
• A legacy hash-entries registry is left untouched — the generator writes the page but prints the entry to add by hand rather than corrupting the array.

Install path + dogfood + docs

• Install templates (doc.rb.erb, docs_kit.rb.erb, installation_page.rb.erb) rewritten to the v2 page DSL + nav_registries + compact class, so install output matches generator output.
• Dogfood Doc + initializer converted to v2. Three titles that don't derive cleanly (Authoring pages → authoring/Authoring, Styling & CSS → styling/Styling, Code languages → languages/Languages) keep explicit slug:/view: overrides so URLs and view classes are preserved. Sidebar is byte-identical before/after.
• README: Configure now leads with nav_registries (the custom lambda demoted to an "advanced" subsection); a new "Add a page" section leads with the generator and reframes hand-authoring as "under the hood". The dogfood authoring page does the same.

Test plan

• spec/docs_kit/registry_spec.rb — page derivation + overrides; all/grouped over page entries; view_class resolution under view_namespace; href; icon; nav_items (authored-only, hrefs/icons); view_namespace-unset → all unauthored; mixed-mode raises.
• spec/docs_kit/configuration_spec.rb — nav_registries default/override; nav_groups derivation; empty-group drop; explicit-nav-wins; empty fallback.
• spec/generators/page_generator_spec.rb — compact class + correct constants; v2 injection + ordering; --slug/--view/--eyebrow/--registry; legacy no-mutation; idempotent re-run + no file clobber.
• Configuration + Registry at 100% line coverage.

Verification

• bundle exec rake — rspec 153 examples, 0 failures + rubocop clean
• Dogfood smoke: rails g docs_kit:page "Smoke Test" --group=Guide → page renders at /docs/smoke-test, appears in sidebar (verified end-to-end over HTTP); reverted.
• All existing dogfood pages render 200 with the derived sidebar; every URL preserved.
• Backwards compatible: the legacy hash-entries API and explicit c.nav lambda both still work.