docs: add llms.txt conceptual documentation to Section 9.18

Add comprehensive llms.txt documentation based on Wasp fullstack essentials
resource evaluation (score 3/5). Sourced from llmstxt.org spec, not the
promotional article.

Changes:
- New section 9.18.4: Documentation Formats for Agents (llms.txt)
- Explains llms.txt standard, format, and use cases
- Clarifies complementarity with Context7 MCP (not opposition)
- Provides minimal and advanced examples with line numbers
- Integration patterns with CLAUDE.md
- References this repo's own llms.txt implementation
- Updated section numbering (9.18.4-9.18.11)
- Updated Section 9.18 TL;DR with new principle
- Added reference.yaml entries for llms.txt

Resource evaluation:
- File: docs/resource-evaluations/wasp-fullstack-essentials-eval.md
- Source: Wasp DevRel blog (framework-agnostic insights extracted)
- Score: 3/5 (partial integration, promotional content excluded)
- Gap identified: Embarrassing to have llms.txt file without explaining concept
- Primary source: llmstxt.org specification

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/resource-evaluations/README.md

@@ -63,7 +63,8 @@ Les documents de travail bruts (prompts Perplexity, audits clients) restent dans
 | **Zolkos** (/insights deep dive) | 4/5 | **4/5** | ✅ Integrate (architecture + facets) | [zolkos-insights-deep-dive.md](./zolkos-insights-deep-dive.md) |
 | **Grenier** (Agent/Skill Quality) | 3/5 | **3/5** | ✅ Intégrer partiellement | [grenier-agent-skill-quality.md](./grenier-agent-skill-quality.md) |
 | **Awesome Claude Skills** (BehiSecc) | 3/5 | **3/5** | ✅ Mention spécialisée | [awesome-claude-skills-github.md](./awesome-claude-skills-github.md) |
+| **Wasp Fullstack Essentials** (Vinny @ Wasp) | 3/5 | **3/5** | ✅ Intégrer concepts framework-agnostiques | [wasp-fullstack-essentials-eval.md](./wasp-fullstack-essentials-eval.md) |
 
 ---
 
-**Dernier update**: 2026-02-07 (55 évaluations)
+**Dernier update**: 2026-02-09 (56 évaluations)