docs: Add YAML front matter to all documentation files

**Purpose:** Prepare documentation for publishing as blog posts or book

**YAML Headers Include:**
- title: Document title
- subtitle: Optional subtitle for context
- description: Brief summary for SEO/indexing
- date: Creation date
- updated: Last update date (for status docs)
- author: Jukka Aho
- categories: Taxonomic classification
- keywords: Search/indexing keywords
- audience: Target reader (users/contributors/researchers)
- level: Difficulty level (beginner/intermediate/advanced/expert)
- type: Document type (manual/guide/theory/benchmark/status)
- series: Which manual it belongs to
- chapter: Book structure (for The JuliaFEM Book)
- status: Current state (completed/work in progress/active maintenance)
- math: Whether document contains mathematical notation
- prerequisites: Required background knowledge
- tools: Software/packages used (for benchmarks)
- context: Background information

**Files Updated:**
- docs/README.md (main index)
- docs/user/README.md (user manual index)
- docs/contributor/README.md (contributor manual index)
- docs/book/README.md (book index)
- docs/contributor/testing_philosophy.md
- docs/contributor/status.md
- docs/contributor/test_fixes_needed.md
- docs/book/lagrange_basis_functions.md
- docs/book/benchmarks/shape_function_derivatives_ad_vs_manual.md
- scripts/README.md

**Benefits:**
- Ready for static site generators (Jekyll, Hugo, MkDocs)
- Can generate book with proper metadata
- SEO-friendly with descriptions and keywords
- Clear audience/level targeting
- Trackable with dates and status
- Organized by series and chapters

**Compatible With:**
- Jekyll (GitHub Pages)
- Hugo (fast static site generator)
- MkDocs (Python-based documentation)
- Jupyter Book (interactive books)
- Docusaurus (React-based docs)
- Custom publishing scripts

scripts/README.md

@@ -1,3 +1,15 @@
+---
+title: "JuliaFEM Scripts"
+description: "Development and code generation scripts for JuliaFEM"
+date: 2025-11-09
+author: "Jukka Aho"
+categories: ["development", "tools", "automation"]
+keywords: ["scripts", "code generation", "automation", "development tools"]
+audience: "contributors"
+level: "intermediate"
+type: "technical documentation"
+---
+
 # JuliaFEM Scripts
 
 This directory contains development and code generation scripts for JuliaFEM.