feat(sterni/blërg/git): render html, org and md entries

Markup for the git backend is implemented as a lookup table to commands
that read markup on stdin and write it to stdout. Markdown uses lowdown,
Org pandoc and HTML doesn't need to be altered.

Future Work:

- Title extraction from the files instead of the file name. Since we use
  the file name as ids in the git backend, it is currently impossible to
  change the title without breaking links.
- Use emacs for Org. Unfortunately, it is really slow, so we may need to
  do something drastic, like writing an sourcegraph style markup
  conversion service or dumping an emacs image.
- Copy post dependencies, e.g. included images.

Change-Id: I88ad4cc18ada1d68d81fdecf685096f105e3fef3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13110
Tested-by: BuildkiteCI
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/blërg/default.nix

@@ -5,8 +5,10 @@ let
   runtimeDependencies = [
     depot.users.sterni.mn2html
     pkgs.mblaze
-    pkgs.execline # execline-cd, importas
+    pkgs.execline # execline-cd, importas, pipeline
     # coreutils   # for printf (assumed to be installed)
+    pkgs.pandoc
+    pkgs.lowdown
   ];
 
   # … and this