docs: Antora component and AsciiDoc migration

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Summary

Adds an Antora documentation component for the Dev-Centr docs portal and migrates existing flat Markdown guides to AsciiDoc.

Docs added/updated

• Antora component (docs/antora.yml, docs/modules/ROOT/)
  • Introduction, delivery model, specification, AI reproduction spec, glass lens capabilities, editor integrations
  • Changelog index with detail pages backfilled from git history
• README.adoc — primary readme with Docs | Dev-Centr badge and CI/Pages badge
• README.md — updated links to docs.devcentr.org/code-lens/
• CHANGELOG.md — unreleased entry for Antora migration
• Cross-repo link updates — package READMEs, extension READMEs, demo App.tsx, implementations/REGISTRY.md

Codepaths covered

• Portable spec and delivery model (spec/, @code-lens/core, @code-lens/vanilla)
• Framework adapter installation patterns
• Glass lens capability tiers and runtime probing
• VS Code / Zed editor integration feasibility
• Release history from v0.1.0 through v0.3.13

Key knowledge gaps addressed

• No published docs site component existed; guides were flat Markdown not wired into the org Antora portal
• README lacked Docs | Dev-Centr badge
• Stale links (docs/*.md, amdphreak.github.io) across demo and package READMEs

Follow-up (cross-repo)

To publish at docs.devcentr.org/code-lens/, add this source to dev-centr/docsantora-playbook.yml:

- url: https://github.com/dev-centr/code-lens.gitbranches: HEADstart_path: docs

Optionally register @antora-supplemental/antora-ai-help-extension on the org playbook for AI-powered search (Lunr is already enabled).