Wrong file name is displayed if non-suffixed file specified in TOC is not found

[lbianchi-lbl]

Noticed while working on #40.

Running idaesx build, the following message was displayed:

[ERROR] 2023-06-02 15:52:16,608 build - During 'pre-process notebooks': Could not find notebook at: /home/ludo/lbl/idaes/examples/idaes_examples/notebooks/docs/tut/general/i_src.ipynb

• The reason for this is that a non-existent docs/tut/general/index.md was specified in _toc.yml (the path should have been docs/tut/index.md)
• However, somewhere in the filename processing logic, the path stem index was processed as though it contained the _doc suffix to be stripped
• The _doc-to-_src logic replaces the last 4 characters regardless of whether the stem contains the suffix or not, resulting in the erroneous i_src.ipynb name being constructed