docs: rewrite documentation.

add: Nix and Snapcraft support.

docs/contributing.mdx

@@ -0,0 +1,50 @@
+---
+sidebar_label: Contributing
+---
+
+# Contributing
+
+## Setup
+
+```bash
+git clone https://github.com/dexoron/dcr
+cd dcr
+cargo build
+```
+
+## Code style
+
+All code must be formatted with `cargo fmt`:
+
+```bash
+cargo fmt
+```
+
+## Linting
+
+```bash
+cargo clippy --all-targets -- -D warnings
+```
+
+## Tests
+
+```bash
+cargo test --all-targets
+```
+
+## PR process
+
+1. Fork the repository
+2. Create a branch: `git checkout -b feature/description`
+3. Make changes
+4. Run `cargo fmt && cargo clippy && cargo test`
+5. Open a Pull Request
+
+## CI
+
+CI runs:
+- `cargo fmt --check`
+- `cargo clippy --all-targets -- -D warnings`
+- `cargo test --all-targets`
+- Build on Linux, macOS, Windows
+- Integration tests