Add markdown files for documentation

projects.xml and declarative-projects.xml were merged with xmllint, and
then I ran that to convert files
for i in *.xml; do pandoc -s -f docbook -t markdown $i -o ${i/xml/md}; done

flake.nix

@@ -162,7 +162,7 @@
           buildInputs =
             [ makeWrapper autoconf automake libtool unzip nukeReferences pkgconfig libpqxx
               gitAndTools.topGit mercurial darcs subversion breezy openssl bzip2 libxslt
-              final.nix perlDeps perl
+              final.nix perlDeps perl mdbook
               boost
               postgresql_11
               (if lib.versionAtLeast lib.version "20.03pre"
@@ -179,8 +179,6 @@
               gzip bzip2 lzma gnutar unzip git gitAndTools.topGit mercurial darcs gnused breezy
             ] ++ lib.optionals stdenv.isLinux [ rpm dpkg cdrkit ] );
 
-          configureFlags = [ "--with-docbook-xsl=${docbook_xsl}/xml/xsl/docbook" ];
-
           shellHook = ''
             PATH=$(pwd)/src/hydra-evaluator:$(pwd)/src/script:$(pwd)/src/hydra-eval-jobs:$(pwd)/src/hydra-queue-runner:$PATH
             PERL5LIB=$(pwd)/src/lib:$PERL5LIB