doc: add a jupyter section

we have many kernels available and nixpkgs should be a good way to
configure software as complex as jupyter.
Right now our API is not straightforward and thus needs documenting.
This is not the final doc, it's an initial PR to bootstrap the effort
and refine the nixpkgs API for jupyter.

future work: document how to wrap jupyter-notebook

doc/packages/index.md

@@ -13,6 +13,7 @@ firefox.section.md
 fish.section.md
 fuse.section.md
 ibus.section.md
+jupyter.section.md
 kakoune.section.md
 linux.section.md
 locales.section.md

doc/packages/jupyter.section.md

@@ -0,0 +1,32 @@
+# jupyter {#sec-jupyter}
+
+Jupyter has different frontends that can be configured: jupyter-console and
+jupyter-notebook. The approach is the same:
+1. create a folder that contains all the jupyter kernels
+2. wrap the executables
+
+```nix
+let
+  definitions = {
+      python = jupyterLib.mkKernelFromPythonEnv (python3.withPackages(ps: [
+      ps.numpy ]);
+      haskell = jupyterLib.mkKernelFromHaskellEnv (pkgs.haskellPackages.ghcWithPackages(hs: [ hs.aeson
+      ]);
+      # ... and all other kernels you are interested in
+  };
+  myKernels = jupyter-kernel.create {
+    inherit definitions;
+  };
+in
+  # pseudocode, real code in next sections
+  jupyter frontend wrapped with JUPYTER_PATH=myKernels
+```
+
+
+## How to run jupyter-console with arbitrary kernels ? {#sec-jupyter-console}
+
+```nix
+myJupyterConsole = jupyter-console.mkConsole {
+  inherit definitions;
+}
+```