docs: generate index.md in Jekyll

This uses a {% for %} loop in Jekyll to render the page, from the "title"
information in the Front Matter of the actual page files.

This also makes `make-index-md` build rule unnecessary, since generation is
done by the template engine itself.

Tested this by running Jekyll locally.

tools/make-index-md.sh

@@ -1,32 +0,0 @@
-#!/bin/sh
-
-set -eu
-
-cd "$@"/docs/
-(
-        echo -e "# systemd Documentation\n"
-
-        for f in *.md ; do
-                if [ "x$f" != "xindex.md" ] ; then
-                        t=`grep "^# " "$f" | head -n 1 | sed -e 's/^#\s*//'`
-
-                        if [ "x$f" = "xCODE_OF_CONDUCT.md" -o "x$f" = "xCONTRIBUTING.md" ] ; then
-                                # For some reason GitHub refuses to generate
-                                # HTML versions of these two documents,
-                                # probably because they are in some way special
-                                # in GitHub behaviour (as they are shown as
-                                # links in the issue submission form). Let's
-                                # work around this limitation by linking to
-                                # their repository browser version
-                                # instead. This might not even be such a bad
-                                # thing, given that the issue submission form
-                                # and our index file thus link to the same
-                                # version.
-                                u="https://github.com/systemd/systemd/blob/master/docs/$f"
-                        else
-                                u="https://systemd.io/"`echo "$f" | sed -e 's/.md$//'`
-                        fi
-                        echo "* [$t]($u)"
-                fi
-        done
-) > index.md