Add MkDocs documentation site

- Add mkdocs.yml with Material theme configuration
- Add docs/ with index, API reference, CLI usage, and examples
- Add GitHub Actions workflow for docs deployment

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

Author: Bissbert

.github/workflows/docs.yml

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+# Reusable workflow: Deploy documentation to GitHub Pages
+# Usage: Copy to each package repo's .github/workflows/
+
+name: Deploy Documentation
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'docs/**'
+      - 'src/**'
+      - 'mkdocs.yml'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - name: Install dependencies
+        run: |
+          pip install mkdocs mkdocs-material mkdocstrings[python]
+
+      - name: Build documentation
+        run: mkdocs build
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: site/
+
+  deploy:
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4

docs/api.md

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+# API Reference
+
+## Core Functions
+
+### parse_cdl
+
+Parse a CDL string into a structured description.
+
+```python
+from cdl_parser import parse_cdl
+
+desc = parse_cdl("cubic[m3m]:{111}@1.0 + {100}@1.3")
+```
+
+::: cdl_parser.parse_cdl
+
+### validate_cdl
+
+Validate a CDL string without parsing.
+
+```python
+from cdl_parser import validate_cdl
+
+is_valid, error = validate_cdl("cubic[m3m]:{111}")
+if not is_valid:
+    print(f"Error: {error}")
+```
+
+::: cdl_parser.validate_cdl
+
+## Data Classes
+
+### CrystalDescription
+
+Main output of CDL parsing.
+
+```python
+@dataclass
+class CrystalDescription:
+    system: str                          # Crystal system
+    point_group: str                     # Point group symbol
+    forms: List[CrystalForm]             # Crystal forms
+    modifications: List[Modification]    # Morphological mods
+    twin: Optional[TwinSpec]             # Twin specification
+```
+
+::: cdl_parser.CrystalDescription
+
+### MillerIndex
+
+Miller index representation.
+
+```python
+@dataclass
+class MillerIndex:
+    h: int
+    k: int
+    l: int
+    i: Optional[int] = None  # For 4-index notation
+
+    def as_tuple(self) -> tuple[int, ...]
+    def as_3index(self) -> tuple[int, int, int]
+```
+
+::: cdl_parser.MillerIndex
+
+### CrystalForm
+
+A crystal form with scale.
+
+```python
+@dataclass
+class CrystalForm:
+    miller: MillerIndex
+    scale: float = 1.0
+    name: Optional[str] = None
+```
+
+::: cdl_parser.CrystalForm
+
+## Constants
+
+```python
+from cdl_parser import (
+    CRYSTAL_SYSTEMS,      # Set of system names
+    POINT_GROUPS,         # Dict[system, Set[groups]]
+    DEFAULT_POINT_GROUPS, # Dict[system, default_group]
+    NAMED_FORMS,          # Dict[name, (h, k, l)]
+    TWIN_LAWS,            # Set of twin law names
+)
+```
+
+### CRYSTAL_SYSTEMS
+
+Set of valid crystal system names:
+
+- `cubic`
+- `tetragonal`
+- `orthorhombic`
+- `hexagonal`
+- `trigonal`
+- `monoclinic`
+- `triclinic`
+
+### POINT_GROUPS
+
+Dictionary mapping each crystal system to its valid point groups.
+
+### DEFAULT_POINT_GROUPS
+
+Dictionary mapping each crystal system to its default (highest symmetry) point group.
+
+### NAMED_FORMS
+
+Dictionary mapping common form names to their Miller indices:
+
+| Name | Miller Index |
+|------|-------------|
+| `octahedron` | {111} |
+| `cube` | {100} |
+| `dodecahedron` | {110} |
+| `trapezohedron` | {211} |
+| `prism` | {10-10} |
+| `basal` | {0001} |
+| `rhombohedron` | {10-11} |
+
+### TWIN_LAWS
+
+Set of recognized twin law names:
+
+- `spinel` - Spinel law (111) twin
+- `brazil` - Brazil law quartz twin
+- `japan` - Japan law quartz twin
+- `fluorite` - Fluorite interpenetration twin
+- `iron_cross` - Iron cross pyrite twin
+
+## Exceptions
+
+### ParseError
+
+Raised when parsing fails due to syntax errors.
+
+```python
+from cdl_parser import ParseError
+
+try:
+    desc = parse_cdl("invalid{{{")
+except ParseError as e:
+    print(f"Syntax error at position {e.position}: {e.message}")
+```
+
+::: cdl_parser.ParseError
+
+### ValidationError
+
+Raised when validation fails due to invalid values.
+
+```python
+from cdl_parser import ValidationError
+
+try:
+    desc = parse_cdl("invalid[xxx]:{111}")
+except ValidationError as e:
+    print(f"Validation error: {e.message}")
+```
+
+::: cdl_parser.ValidationError