Test, harden and document AI Figma: concise API errors, docs site + GitLab Pages, remove dead config

Problem/Motivation

The 1.0.x scaffold from #3608437 needs real-world verification, a self-contained codebase (no leftover references to modules/features that don't exist), and published documentation before it can be considered reviewable.

Steps to reproduce

N/A – hardening/testing/documentation task, not a bug.

Proposed resolution

1. Built a fresh Drupal CMS ~2.0 site and required drupal/ai_figma:1.0.x-dev; ran 10 Playwright test iterations covering the settings page, status report, permissions, SSRF validation, a live Figma 403 response, AI tool execution, and anonymous access.
2. Hardened Figma HTTP error handling to a concise 403 Invalid token-style message instead of a raw exception dump.
3. Rewrote README.md to match the actual, minimal module (removed aspirational/inaccurate claims).
4. Added a docs/ MkDocs site plus a GitLab CI pages job publishing to https://project.pages.drupalcode.org/ai_figma/.
5. Removed vestigial builder-only config (layouts, component_choices, content_roles, builder_order, default_layout, component_prefix) from ai_figma.settings.yml and its schema.
6. Cleaned up code comments and the .info.yml description.
7. Install now seeds the editable Figma AI Context items when ai_context is present.

AI usage disclosure: Assisted by Claude Code (Anthropic) for the testing pass, hardening, docs and cleanup described above. All AI-generated code is human-reviewed before merge.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ✅ Automated unit/functional testing coverage
• ✅ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ✅ Reviewed by a human
• ✅ Code review by maintainers
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes
• ✅ Release

User interface changes

• No new admin UI. The existing settings page loses the vestigial builder-only fields being removed from config/schema.

API changes

• Hardens FigmaContextClient error handling to return concise, user-facing HTTP error messages (e.g. 403 Invalid token). No new services or plugins.

Data model changes

• Removes the builder-only keys (layouts, component_choices, content_roles, builder_order, default_layout, component_prefix) from the ai_figma.settings config object and schema. Install now seeds default, editable Figma AI Context items when ai_context is present.

Release notes snippet

• Verified the AI Figma 1.0.x scaffold end-to-end on a fresh Drupal CMS site, tightened Figma API error messages, published README + a MkDocs documentation site (GitLab Pages), and removed dead builder-only configuration.