docs: rewrite documentation.

add: Nix and Snapcraft support.

docs/testing/running-tests.mdx

@@ -0,0 +1,45 @@
+---
+sidebar_label: Running tests
+---
+
+# Running Tests
+
+## Execution
+
+```bash
+dcr test
+```
+
+Builds the project, then compiles and runs test files from `tests/`.
+
+## Profiles
+
+```bash
+dcr test                     # debug (default)
+dcr test --release           # release
+dcr test --debug             # explicit debug
+```
+
+## Output
+
+```
+=====================
+  Testsuite summary
+=====================
+TOTAL: 5
+PASS: 3
+SKIP: 1
+FAIL: 1
+=====================
+```
+
+## Exit code
+
+- 0 — all tests passed (FAIL = 0)
+- 1 — test failures or build error
+
+## What gets built
+
+- All `.c` files from `tests/` (`.cpp` not supported)
+- Include path: `tests/` (for `dcr_test.h`)
+- Linked with project if `package.type = "lib"` or `kind = "staticlib"`/`"sharedlib"`

docs/testing/test-framework.mdx

@@ -0,0 +1,68 @@
+---
+sidebar_label: Test framework (EXPECT, TEST, ...)
+---
+
+# Test Framework
+
+DCR has a built-in minimal test framework for C.
+
+## Macros
+
+### `EXPECT(expr)`
+
+Asserts that an expression is true.
+
+```c
+EXPECT(1 + 1 == 2);
+EXPECT(ptr != NULL);
+```
+
+### `SKIP(reason)`
+
+Skips a test with a message.
+
+```c
+SKIP("not implemented on Windows");
+```
+
+### `TEST(name)`
+
+Defines a test.
+
+```c
+TEST(addition) {
+    EXPECT(1 + 1 == 2);
+    EXPECT(2 + 2 == 4);
+}
+```
+
+### `TEST_CASE(name)`
+
+Registers a test case.
+
+```c
+TEST_CASE(math) {
+    EXPECT(1 + 1 == 2);
+}
+```
+
+## Initialization
+
+```bash
+dcr test --init
+```
+
+Creates:
+- `tests/dcr_test.h` — framework header (do not edit)
+- `tests/test.c` — template with example test
+
+## Structure
+
+```
+tests/
+├── dcr_test.h      # framework (do not edit)
+├── test.c          # main tests
+└── ...             # additional .c test files
+```
+
+Only `.c` files are compiled (`.cpp` is not supported).