docs: add comprehensive configuration reference and usage guides

- Add detailed configuration reference documentation covering all
  configuration layers, environment variables, and TOML file options
  including LLM settings, retry mechanisms, fallback strategies,
  retrieval configurations, and metrics tracking

- Create Python usage guide with installation instructions,
  configuration methods (zero config, custom settings, config files),
  indexing examples from various sources (files, content, bytes),
  querying techniques, and document management operations

- Develop Rust usage guide featuring engine builder patterns,
  multiple configuration approaches (presets, custom endpoints),
  indexing workflows with different input types and options,
  advanced querying with retrieve options and strategy preferences,
  streaming capabilities, and document graph functionality

- Include comprehensive examples section for both Python and Rust
  with links to example repositories demonstrating various use cases

Author: zTgx

docs/design/lovable-vectorless.png

@@ -0,0 +1,218 @@
+# Configuration Reference
+
+Vectorless supports multiple configuration layers. Settings from higher-priority sources override lower ones.
+
+## Priority (highest → lowest)
+
+1. Builder/constructor parameters
+2. Environment variables
+3. Explicit config file (`config_path`)
+4. Auto-detected config file (`vectorless.toml`, `config.toml`, `.vectorless.toml`)
+5. Default values
+
+## Environment Variables
+
+| Variable | Description | Example |
+|----------|-------------|---------|
+| `OPENAI_API_KEY` | LLM API key | `sk-...` |
+| `VECTORLESS_MODEL` | Default model | `gpt-4o-mini` |
+| `VECTORLESS_ENDPOINT` | Custom API endpoint | `https://api.openai.com/v1` |
+| `VECTORLESS_WORKSPACE` | Workspace directory | `./data` |
+
+## Configuration File
+
+Create `vectorless.toml` for full control:
+
+```bash
+cp vectorless.example.toml ./vectorless.toml
+```
+
+### LLM Configuration
+
+Three separate LLM clients for different tasks:
+
+```toml
+[llm]
+api_key = "sk-your-api-key"
+
+# Summarization — used during indexing
+[llm.summary]
+model = "gpt-4o-mini"
+endpoint = "https://api.openai.com/v1"
+max_tokens = 200
+temperature = 0.0
+
+# Retrieval — used during query analysis
+[llm.retrieval]
+model = "gpt-4o"
+endpoint = "https://api.openai.com/v1"
+max_tokens = 100
+temperature = 0.0
+
+# Pilot — used for tree navigation guidance
+[llm.pilot]
+model = "gpt-4o-mini"
+endpoint = "https://api.openai.com/v1"
+max_tokens = 300
+temperature = 0.0
+```
+
+### Retry and Rate Limiting
+
+```toml
+[llm.retry]
+max_attempts = 3
+initial_delay_ms = 500
+max_delay_ms = 30000
+multiplier = 2.0
+retry_on_rate_limit = true
+
+[llm.throttle]
+max_concurrent_requests = 10
+requests_per_minute = 500
+enabled = true
+```
+
+### Fallback
+
+```toml
+[llm.fallback]
+enabled = true
+models = ["gpt-4o-mini", "glm-4-flash"]
+on_rate_limit = "retry_then_fallback"
+on_timeout = "retry_then_fallback"
+on_all_failed = "return_error"
+```
+
+### Retrieval Configuration
+
+```toml
+[retrieval]
+top_k = 3
+max_tokens = 1000
+
+[retrieval.search]
+top_k = 5
+beam_width = 3
+max_iterations = 10
+min_score = 0.1
+
+[retrieval.sufficiency]
+min_tokens = 500
+target_tokens = 2000
+max_tokens = 4000
+confidence_threshold = 0.7
+
+[retrieval.cache]
+max_entries = 1000
+ttl_secs = 3600
+```
+
+### Strategy Configuration
+
+```toml
+# Hybrid (BM25 + LLM)
+[retrieval.strategy.hybrid]
+enabled = true
+pre_filter_ratio = 0.3
+bm25_weight = 0.4
+llm_weight = 0.6
+
+# Cross-document search
+[retrieval.strategy.cross_document]
+enabled = true
+max_documents = 10
+max_results_per_doc = 3
+merge_strategy = "TopK"
+parallel_search = true
+
+# Page range (PDF)
+[retrieval.strategy.page_range]
+enabled = true
+include_boundary_nodes = true
+expand_context_pages = 0
+```
+
+### Pilot Configuration
+
+```toml
+[pilot]
+mode = "Balanced"        # "Conservative", "Balanced", "Aggressive"
+guide_at_start = true
+guide_at_backtrack = true
+
+[pilot.budget]
+max_tokens_per_query = 2000
+max_tokens_per_call = 500
+max_calls_per_query = 5
+
+[pilot.intervention]
+fork_threshold = 3
+score_gap_threshold = 0.15
+low_score_threshold = 0.3
+```
+
+### Multi-turn Retrieval
+
+```toml
+[retrieval.multiturn]
+enabled = true
+max_sub_queries = 3
+decomposition_model = "gpt-4o-mini"
+aggregation_strategy = "merge"
+```
+
+### Reference Following
+
+```toml
+[retrieval.reference]
+enabled = true
+max_depth = 3
+max_references = 10
+follow_pages = true
+follow_tables_figures = true
+min_confidence = 0.5
+```
+
+### Storage Configuration
+
+```toml
+[storage]
+workspace_dir = "./workspace"
+cache_size = 100
+atomic_writes = true
+file_lock = true
+checksum_enabled = true
+
+[storage.compression]
+enabled = false
+algorithm = "gzip"
+level = 6
+```
+
+### Metrics
+
+```toml
+[metrics]
+enabled = true
+storage_path = "./workspace/metrics"
+retention_days = 30
+
+[metrics.llm]
+track_tokens = true
+track_latency = true
+track_cost = true
+cost_per_1k_input_tokens = 0.00015
+cost_per_1k_output_tokens = 0.0006
+
+[metrics.pilot]
+track_decisions = true
+track_accuracy = true
+track_feedback = true
+
+[metrics.retrieval]
+track_paths = true
+track_scores = true
+track_iterations = true
+track_cache = true
+```