Changed documentation of external_exporters

[JimZwartveld]

The example provided in the documentation required the user to append his/her path to templates to super().template_paths, while super().template_paths is looking for the user provided template name.

Changed documentation such that user updates extra_template_basedirs.

I believe this PR can close Issue #1581

[amichuda]

Hi! I'm writing a custom exporter from .ipynb to Stata .do files and I changed the code accordingly, and now I get:

ValueError: No template sub-directory with name 'dofile' found in the following paths:
        /home/<user>/.local/share/jupyter
        /usr/local/share/jupyter
        /usr/share/jupyter

I believe that the issue might lie in:

nbconvert/nbconvert/exporters/templateexporter.py

Lines 577 to 584 in da5bb65

	for base_dir in self.extra_template_basedirs:
	template_dir = os.path.join(base_dir, base_template)
	if os.path.exists(template_dir):
	found_at_least_one = True
	conf_file = os.path.join(template_dir, 'conf.json')
	if os.path.exists(conf_file):
	with open(conf_file) as f:
	conf = recursive_update(json.load(f), conf)

where found_at_least_one is never flagged as True because there's a path check, and it's not checked with its extension. Could that be the case.

In any case, if that check goes through by changing template_dir to template_dir + ".tpl", I get this weird error now

...
  File "/usr/lib/python3.9/site-packages/jinja2/loaders.py", line 556, in load
    raise TemplateNotFound(name)
jinja2.exceptions.TemplateNotFound: index.<extension>.j2

Is this a bug or there's something wrong with my understanding?