Improve documentation on how to customize the user environment

I think the AiiDAlab docker stack has the potential to eventually become the go-to solution for creating and a customized AiiDA environment and distributing it across an organization (research group, company, ...).

However, in basically every such case the administrators of such an environment will need to customize it to fit their needs.
This includes the installation of custom packages at the image level, but it also includes setup steps at the container level - for the latter I'm thinking of

- extra packages that should be installed in editable mode in the user's home directory so they are easy to work on
- extra computers/codes to be set up

It would therefore be very valuable to add a short section "Customization" to the [administrator guide](https://aiidalab.readthedocs.io/en/latest/admin/index.html) that goes through the basic stages of the setup of the image & container and points out the specific places where to inject customization (e.g. I guess there will be some places in the Dockerfile + some folders where one can place startup scripts).

@yakutovicha @csadorf Would any of you be able to have a look into this?