nixos/doc/md-to-db.sh: consistent pandoc version

Until now, this script has used the version of pandoc from unstable.
This means that running the script on the same version of Nixpkgs
could produce different results, and meant that when Pandoc's output
was changed, random PRs were changing the whole manual when they ran
the script to regenerate docs[1][2].

Here I've changed the manual to use a consistent version of pandoc —
the one from the latest release tag, which will avoid this problem in
future.  This will avoid this problem in future.  The only time we'll
need to worry about pandoc output changes is when we bump the version
used in this script.

I also considered using the version from the current Nixpkgs branch,
but decided against it as it's unlikely that e.g. the person bumping
Pandoc will remember to regenerate the manual.

[1]: https://github.com/NixOS/nixpkgs/pull/162550
[2]: https://github.com/NixOS/nixpkgs/pull/168535

nixos/doc/manual/md-to-db.sh

@@ -1,5 +1,5 @@
 #! /usr/bin/env nix-shell
-#! nix-shell -I nixpkgs=channel:nixpkgs-unstable -i bash -p pandoc
+#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/tarball/21.11 -i bash -p pandoc
 
 # This script is temporarily needed while we transition the manual to
 # CommonMark. It converts the .md files in the regular manual folder