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.

Start by tracing how settings.ini currently reads and applies nbs_path during the nbdev documentation build. Compare the requested multiple-path and exclude_pattern forms, and consider the work done when the centralized build includes the tutorial notebooks while omitting the external libraries' core directories.

Written by the indexing model from the issue text.