docs: adopt a text-first static layout

docs/src/index.md

@@ -1,4 +1,4 @@
-# Kaptan Developer Manual
+# Kaptan documentation
 
 Kaptan is an embedded Rust flight-controller project. This manual is the
 single source of project documentation: board support, safety boundaries,
@@ -9,7 +9,7 @@ The checked-in firmware is telemetry-only. It does not expose physical motor
 authority, and no document in this site should be read as a physical-flight
 release claim.
 
-## Start here
+## Index
 
 - New contributors should read [Getting started](getting-started.md).
 - Board work belongs under [Boards](boards/index.md).