fixed some style issues, added tooling for docs

docs/core/timer.md

@@ -0,0 +1,13 @@
+# core/timer.h
+
+## class Timer — public interface
+
+### `Timer() { ... }`
+### `init();`
+### `void init() noexcept { ... }`
+### `_start = clock::now();`
+### `Time start() const noexcept { ... }`
+### `auto tp = _start.time_since_epoch();`
+### `Time now() const noexcept { ... }`
+### `auto tp = clock::now().time_since_epoch();`
+### `Time elapsed() const noexcept { ... }`