Add section to customizing showing how to use template inheritance

[stefanv]

@SylvainCorlay @choldgraf An update to the docs to illustrate new-style template usage, as suggested :)

@Carreau Since this is based on your original template from https://github.com/Carreau/jupytercontrib

Note: there are still entire sections in the docs that need reviewing for new-style templates, such as https://nbconvert.readthedocs.io/en/latest/external_exporters.html

[choldgraf]

@Carreau can you add me as an admin on ReadTheDocs? Or can you enable PR previews?

[choldgraf]

I can't enable the readthedocs PR previews because I'm not an admin there, but the copy in general looks good to me!

[SylvainCorlay]

Thanks!