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"`