notebooks with auto in their name are skipped in doclinks

# Provide a minimally reproducible example

#1428 changed the following line

https://github.com/AnswerDotAI/nbdev/blob/8095d12c260dd7cc5d8c2ca34e89819c58fd679a/nbdev/doclinks.py#L74

to

https://github.com/AnswerDotAI/nbdev/blob/ea935f5cb6adf3f1206dc55f3b6256a7ec3e14a7/nbdev/doclinks.py#L80

which makes `nbdev_export` remove all the syms in `_modidx.py` for a notebook that has `auto` in its name, see for example the following PRs:

* https://github.com/Nixtla/mlforecast/pull/392/commits/4df35c74e86937fd9c2c5e973ed95ef894dfb001
* https://github.com/Nixtla/neuralforecast/pull/1085/commits/02db6808510d88913d2bd04abed8a3930766afa4

Inizia in nbdev/doclinks.py alla riga indicata e confronta le modifiche di #1428; riproduci il problema con un notebook il cui nome contiene `auto` ed esegui `nbdev_export`. Il lavoro è completato quando `_modidx.py` conserva i simboli di quel notebook invece di rimuoverli, come mostrato nei commit collegati di mlforecast e neuralforecast.

Scritto dal modello di indicizzazione a partire dal testo della issue.