manual: remove "Writing Nix Expressions" chapter

it is out of date, all over the place in level of detail, is really
about `nixpkgs`, and in general instructions should not be part of
a reference manual.

also:
- update redirects and internal links
- use "Nix language" consistently

doc/manual/src/command-ref/nix-store.md

@@ -121,7 +121,7 @@ Special exit codes:
   - `102`\
     Hash mismatch, the build output was rejected because it does not
     match the [`outputHash` attribute of the
-    derivation](../expressions/advanced-attributes.md).
+    derivation](../language/advanced-attributes.md).
 
   - `104`\
     Not deterministic, the build succeeded in check mode but the