chore: documentation (#76)

* chore: update docstring example to use python code block

* docs: add documentation

* feat: add docs build + fix other workdlows

* fix: add missing return type

docs/reference/metrics.md

@@ -0,0 +1,15 @@
+# `metrics`
+
+Here's the reference for the Prometheus metrics registry and endpoint handler.
+
+You can import them directly from `fastapi_toolsets.metrics`:
+
+```python
+from fastapi_toolsets.metrics import Metric, MetricsRegistry, init_metrics
+```
+
+## ::: fastapi_toolsets.metrics.registry.Metric
+
+## ::: fastapi_toolsets.metrics.registry.MetricsRegistry
+
+## ::: fastapi_toolsets.metrics.handler.init_metrics