Add initial project structure and core functionality for ArchDoc

- Created `.gitignore` files for various directories to exclude unnecessary files.
- Added `PLAN.md` to outline the project goals and architecture documentation generation.
- Implemented the `archdoc-cli` with a command-line interface for initializing and generating documentation.
- Developed the `archdoc-core` library for analyzing Python projects and generating architecture documentation.
- Included caching mechanisms to optimize repeated analysis.
- Established a comprehensive test suite to ensure functionality and error handling.
- Updated `README.md` to provide an overview and installation instructions for ArchDoc.

archdoc-core/src/errors.rs

@@ -0,0 +1,26 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum ArchDocError {
+    #[error("IO error: {0}")]
+    Io(#[from] std::io::Error),
+    
+    #[error("Parse error in {file}:{line}: {message}")]
+    ParseError {
+        file: String,
+        line: usize,
+        message: String,
+    },
+    
+    #[error("Configuration error: {0}")]
+    ConfigError(String),
+    
+    #[error("Analysis error: {0}")]
+    AnalysisError(String),
+    
+    #[error("Rendering error: {0}")]
+    RenderingError(String),
+    
+    #[error("File consistency check failed: {0}")]
+    ConsistencyError(String),
+}