Rename doc/manual{src -> source}

This is needed to avoid this
https://github.com/mesonbuild/meson/issues/13774 when we go back to
making our subproject directory `src`.

doc/manual/source/language/builtins-prefix.md

@@ -0,0 +1,18 @@
+# Built-ins
+
+This section lists the values and functions built into the Nix language evaluator.
+All built-ins are available through the global [`builtins`](#builtins-builtins) constant.
+
+Some built-ins are also exposed directly in the global scope:
+
+<!-- TODO(@rhendric, #10970): this list is incomplete -->
+
+- [`derivation`](#builtins-derivation)
+- [`import`](#builtins-import)
+- [`abort`](#builtins-abort)
+- [`throw`](#builtins-throw)
+
+<dl>
+  <dt id="builtins-derivation"><a href="#builtins-derivation"><code>derivation <var>attrs</var></code></a></dt>
+  <dd><p><var>derivation</var> is described in
+         <a href="derivations.md">its own section</a>.</p></dd>