doc: explicit Markdown anchors for top-level headings; remove metadata

I used the existing anchors generated by Docbook, so the anchor part
should be a no-op. This could be useful depending on the
infrastructure we choose to use, and it is better to be explicit than
rely on Docbook's id generating algorithms.

I got rid of the metadata segments of the Markdown files, because they
are outdated, inaccurate, and could make people less willing to change
them without speaking with the author.

doc/preface.chapter.md

@@ -1,10 +1,4 @@
----
-title: Preface
-author: Frederik Rietdijk
-date: 2015-11-25
----
-
-# Preface
+# Preface {#preface}
 
 The Nix Packages collection (Nixpkgs) is a set of thousands of packages for the
 [Nix package manager](https://nixos.org/nix/), released under a