docs(vc-shell): scaffold VC-Shell docs section (infrastructure + manual content)

[maksimzinchuk]

Summary

Land the infrastructure and manual narrative content for the VC-Shell developer documentation under custom-apps-development/vc-shell/. The auto-synced framework reference docs (components, composables, reference/api, reference/modules) are intentionally NOT in this PR — they are delivered by the sync-docs.yml CI workflow on each vc-shell release, which opens a separate PR labeled auto-generated.

What's in this PR

Commit 1 — Infrastructure

• mkdocs.yml (developer-guide): single awesome-pages rest pattern ... | regex=^custom-apps-development/.+\.md$ replaces the previous 75-line manual enumeration of VC-Shell pages.
• vc-shell-docs.css: Storybook iframe embeds, admonition styling, table tweaks, frozen-version banner.
• overrides/main.html: theme hook for the frozen-version banner.
• .github/workflows/protect-generated.yml: CI guard that blocks human edits to AUTO-GENERATED files. Skips manual folders (introduction/, getting-started/, guides/, reference/cli/, reference/migration/). Bypassed for vc-shell-docs-bot and PRs labeled auto-generated.

Commit 2 — Manual narrative content

• introduction/ — overview, what-is-vc-shell, architecture-overview, when-to-use
• getting-started/ — installation, create-your-app, project-structure, connecting-to-platform, first-blade
• concepts/ — narrative stubs for blade-navigation, modules, extensions, layout, permissions-model, localization, state-persistence, api-clients
• guides/ — placeholder index pages for blades, cookbook, data, forms, modules-and-extensions, platform, troubleshooting, ui
• reference/cli/ — api-client-generator and create-vc-app
• reference/migration/ — placeholder
• reference/api/index.md and reference/modules/index.md — placeholder index pages (the AUTO siblings come from CI)
• vc-shell/.pages and custom-apps-development/.pages — manual nav scaffolding (excludes components/composables/plugins entries — added in follow-up docs(vc-shell): add components, composables, plugins to nav order #59 after CI auto-sync populates the folders)

Also removes the legacy hand-curated VC-Shell pages from the previous layout (Essentials/Usage-Guides/Plugins/Shared/UI Components, Extensibility, How-tos, capitalized Getting-started/Guides).

Sequence

1. This PR (docs(vc-shell): scaffold VC-Shell docs section (infrastructure + manual content) #57) merges → infrastructure + manual content land on main. Site builds fine; components/, composables/, plugins/ paths 404 until CI auto-sync runs.
2. vc-shell release cut (post #223 merge) → sync-docs.yml opens an auto/sync-vc-shell-<tag> PR with all AUTO content (~147 files in components/, composables/, reference/api/, reference/modules/, plugins/). Reviewer merges with the auto-generated label bypassing the guard.
3. PR docs(vc-shell): add components, composables, plugins to nav order #59 (draft) marked ready, merged → adds components/composables/plugins to vc-shell/.pages nav order. Final sidebar takes shape.

Companion PRs

• vc-shell #223 (merged into vc-shell main): introduces the plugins category in docs-sync.
• vc-docs docs(vc-shell): add components, composables, plugins to nav order #59 (draft, blocked on first CI auto-sync): adds components/composables/plugins entries to manual vc-shell/.pages nav order.

Test plan

• mkdocs build -f platform/developer-guide/mkdocs.yml — exit 0
• mkdocs serve — sidebar shows Introduction / Getting Started / Concepts / Guides / Reference with manual stubs. Components/Composables/Plugins absent (expected; CI populates them).
• CI green

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}