Specify multiple directories in `nbs_path` or exclude certain subdirectories

I would like to build nbdev docs (nb: docs only, no lib code) based on multiple subfolders - the pattern is as follows:

```
repo_root
└ nbs_dir
├ sub-directory1
│ ├ core/
│ └ tutorials/nbs/... # here are the tutorials!
├ sub-directory2
│ ├ core/
│ └ tutorials/nbs/... # here are the tutorials!
└ sub-directory3
├ core/
└ tutorials/nbs/... # here are the tutorials!

```

Thus, I'd like to specify my `nbs_path` in `settings.ini` either as follows:

**Option 1**
```
# settings.ini
nbs_path = nbs_dir/sub-directory1/tutorials, nbs_dir/sub-directory2/tutorials, nbs_dir/sub-directory3/tutorials
```

**Option 2**
```
# settings.ini
nbs_path = nbs_dir
exclude_pattern = nbs_dir/*/core/
```

# Additional context:
sub-dir1/2/3 are submodules from external libraries which have their own lib code and nbdev tutorials.
This repo would only serve as synced, centralised point to access all tutorials in a single build.

Inizia tracciando come settings.ini legge e applica attualmente nbs_path durante la build della documentazione nbdev. Confronta le forme richieste per più percorsi ed exclude_pattern, e considera il lavoro svolto quando la build centralizzata include i notebook del tutorial omettendo le directory core delle librerie esterne.

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