docs: rewrite documentation.

add: Nix and Snapcraft support.

docs/reference/cross-compilation.mdx

@@ -0,0 +1,60 @@
+---
+sidebar_label: Cross-compilation
+---
+
+# Cross-Compilation
+
+## Short Names
+
+DCR supports short platform names:
+
+| Short Name | Full Triple |
+|-----------|-------------|
+| `linux` | `x86_64-unknown-linux-gnu` |
+| `macos` | `x86_64-apple-darwin` |
+| `windows` | `x86_64-pc-windows-msvc` |
+
+```bash
+dcr build --target windows
+```
+
+## Full Triples
+
+```bash
+dcr build --target aarch64-unknown-linux-gnu
+dcr build --target x86_64-pc-windows-gnu      # mingw
+dcr build --target armv7-unknown-linux-gnueabihf
+```
+
+## clang --target
+
+When using clang, DCR injects `--target=<triple>` into CFLAGS.
+
+```toml
+[build]
+compiler = "clang"
+target = "aarch64-unknown-linux-gnu"
+# Auto: cflags += ["--target=aarch64-unknown-linux-gnu"]
+```
+
+## Bare-Metal
+
+For targets containing `none`, `-elf`, `eabi`, or `baremetal`, DCR default flags are disabled (no system include paths, no `-l` libc).
+
+```bash
+dcr build --target aarch64-none-elf
+```
+
+## Target Directory
+
+On Linux with explicit target:
+
+```
+target/<triple>/<profile>/
+```
+
+On macOS/Windows or without target:
+
+```
+target/<profile>/
+```