docs: add doc 'Configuration and maintenance'

- Move section `updating` from `usage.md` to `configuration.md` and
  rename `usage.md` -> `services.md`.
  `services.md` documents how enable and interact with node services.

- README: Move `docs` below `Get started`. The `docs` section is short and
  should be easily accessible.

README.md

@@ -47,6 +47,14 @@ Get started
 - To add nix-bitcoin to an existing NixOS configuration, see [importable-configuration.nix](examples/importable-configuration.nix)
   and the [Flake example](examples/flakes/flake.nix).
 
+Docs
+---
+* [Hardware Requirements](docs/hardware.md)
+* [Installation](docs/install.md)
+* [Configuration and maintenance](docs/configuration.md)
+* [Using services](docs/services.md)
+* [FAQ](docs/faq.md)
+
 Features
 ---
 A [configuration preset](modules/presets/secure-node.nix) for setting up a secure node
@@ -92,13 +100,6 @@ Security
 
 Note that if the machine you're deploying *from* is insecure, there is nothing nix-bitcoin can do to protect itself.
 
-Docs
----
-* [Hardware Requirements](docs/hardware.md)
-* [Install instructions](docs/install.md)
-* [Usage instructions](docs/usage.md)
-* [FAQ](docs/faq.md)
-
 Troubleshooting
 ---
 If you are having problems with nix-bitcoin check the [FAQ](docs/faq.md) or submit an issue.\