docs(config): add CONFIG.md and move the configuration reference out of the README (#119)

CONFIG.md documents every section of sprout.toml, the command line options, BLS support and strict mode. The README keeps the overview and minimal examples and links to it. Adds an update-config-docs skill to keep the reference in sync with the code.

crates/config/src/lib.rs

@@ -97,7 +97,7 @@ pub struct OptionsConfiguration {
     /// Makes Sprout follow the Boot Loader Specification and systemd-boot exactly where it
     /// otherwise differs for convenience or safety. For example, the extended boot loader
     /// partition is always read, a one-shot entry only sets the default entry, and entries with
-    /// missing files are hidden. See the README for everything it changes.
+    /// missing files are hidden. See CONFIG.md for everything it changes.
     #[serde(rename = "bls-strict-mode", default)]
     pub bls_strict_mode: bool,
 }