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it is out of date, all over the place in level of detail, is really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/language/builtins-prefix.md

@@ -0,0 +1,16 @@
+# Built-in Functions
+
+This section lists the functions built into the Nix expression
+evaluator. (The built-in function `derivation` is discussed above.)
+Some built-ins, such as `derivation`, are always in scope of every Nix
+expression; you can just access them right away. But to prevent
+polluting the namespace too much, most built-ins are not in
+scope. Instead, you can access them through the `builtins` built-in
+value, which is a set that contains all built-in functions and values.
+For instance, `derivation` is also available as `builtins.derivation`.
+
+<dl>
+  <dt><code>derivation <var>attrs</var></code>;
+      <code>builtins.derivation <var>attrs</var></code></dt>
+  <dd><p><var>derivation</var> is described in
+         <a href="derivations.md">its own section</a>.</p></dd>