docs: Improve homepage, navigation to be more obvious

Change-Id: I6a6a6964debc026916656181c9b0bdd5c2622069

docs/modules/ROOT/nav.adoc

@@ -1,10 +1,5 @@
 * xref:index.adoc[]
 
-* Core concepts
-** xref:core/derivations.adoc[]
-** xref:core/zexp.adoc[]
-** xref:core/vfs.adoc[]
-** xref:core/zilch-as-lib.adoc[]
 * Language support
 ** Go
 *** xref:go/usage.adoc[]
@@ -18,5 +13,11 @@
 *** xref:ninja/usage.adoc[]
 *** xref:ninja/man.adoc[Man page]
 
+* Core concepts
+** xref:core/derivations.adoc[]
+** xref:core/zexp.adoc[]
+** xref:core/vfs.adoc[]
+** xref:core/zilch-as-lib.adoc[]
+
 * Code reference
 include::generated:partial$nav.adoc[]

docs/modules/ROOT/pages/index.adoc

@@ -13,6 +13,10 @@ Features:
 * Batteries included
 * Intercompatible with Nixpkgs and other, arbitrary, Nix expressions.
 
+Its current focus is on incremental builds for xref:go/usage.adoc[Go],
+xref:rust/usage.adoc[Rust], and xref:ninja/usage.adoc[Meson/CMake-based C/C++
+projects].
+
 == Contributing
 
 Come join [.line-through]#us# me at `#zilch` on https://libera.chat[libera.chat]!