tiny improvements to documentation

docs/installation/configuration.md

@@ -0,0 +1,31 @@
+# Configuration
+
+While RustiCal (apart from user management) will work without any configuration you should still know how to configure it. :)
+
+You can either mount a `config.toml` file or use environment variables (recommended).
+
+To see the options you can generate a default configuration using
+
+```sh title="Generate default config.toml"
+rustical gen-config
+```
+
+To see all configuration options available you can browse the [Cargo docs](/rustical/_crate/rustical/config/struct.Config.html).
+
+## Environment variables
+
+The options in `config.toml` can also be configured using environment variables.
+Names translate the following:
+
+```toml title="Example config.toml"
+[data_store.toml]
+path = "asd"
+```
+
+becomes `RUSTICAL_DATA_STORE__TOML__PATH`.
+Every variable is
+
+- uppercase
+- prefixed by `RUSTICAL_`
+- Dots become `__`
+- Arrays are JSON-encoded