Improve docs on customising the hub image

[manics]

See https://discourse.jupyter.org/t/can-i-use-hub-initcontainers-or-hub-extracontainers-to-add-packages-that-would-then-be-visible-to-jupyterhub/2033

These are the current customisation topics in the docs:

Customization Guide

    Customizing your Deployment
        Applying configuration changes
    Customizing User Environment
    Customizing User Resources
    Customizing User Storage
    Customizing User Management

Adding a section on customising the hub image might be helpful?

[consideRatio]

Related #1464