fix: allow custom template files outside of the template system

[maartenbreddels]

As discussed over a videocall with @MSeal and @SylvainCorlay, this PR:

• Fixes TemplateNotFound error when extending built-in template #1394
• is an alternative to a part of Fixed template assignments with old basic and full names with tpl extension #1387
• and is a followup of DOC: Remove outputs from example notebooks #1292

In #1292, we kind of 'tricked' the template system, by replacing the template_name by the parent directory name of the template_file. That caused an override of the template name. If we don't do that, but simply add that directory to the template search path, the whole template system machinery works with us. This allows .e.g:

$ python -m nbconvert --to html --execute --template material --TemplateExporter.template_file=nbconvert/exporters/tests/files/lablike.html.j2 ~/src/QuantStack/voila/notebooks/basics.ipynb

So we use a particular template (material in this case, but could be labl) but override the template_file.

[MSeal]

Thanks! I tested some of the reported issues. We should probably also do a PR to update documentation for backwards compatibility. I'll get the basic template PR up later today to wrap up the other half of the implementation from the larger PR.