fixed some style issues, added tooling for docs

docs/core/error.md

@@ -0,0 +1,9 @@
+# core/error.h
+
+## Free functions
+
+### `std::mutex &error_mutex() noexcept;`
+### `inline T&& show(std::string_view name, T&& value) noexcept { ... }`
+### `std::lock_guard<std::mutex> lock(error_mutex());`
+### `inline T&& eval_and_show(std::string_view expr, T&& value) noexcept { ... }`
+### `std::lock_guard<std::mutex> lock(error_mutex());`