Add a configuration for secnumdepth/toc that is accessible via the LatexPreprocessor

[Uripeer3]

Hello,
I have seen a lot of requests for removing Section Numbering or adding a Table of Contents when using pdf conversion (or LaTeX).

The only solution I have seen so far is with a custom template.
Yet this seems to be too common a problem to demand this solution (the average academic user doesn't necessarily know to create his own template).

I propose that, similarly to the manner LatexPreprocessor works for author and title, we could add an injection
LatexPreprocessor.secnumdepth

This could add to the notebook metadata, for example, a:
'latex' : {'secnumdepth' : 0}

which can be later inferred from the Jinja Template.
This can be easily called from the CLI using:

jupyter nbconvert --to pdf {notebook_name}.ipynb \
--output-dir /content/ \
--LatexPreprocessor.title "{report_title}"  \
--LatexPreprocessor.secnumdepth "0" \
--LatexPreprocessor.toc True

etc...

This will additionally open up a connection between the metadata and the Jinja template that can be exploited additionally by template creators, and open up a more dynamic templates interface that can also be accesible form the CLI.
*For the wide use-case of secnumdepth, we would need to slightly modify the base template.

Concretely:
Edit LatexPreprocessor to include a general property injector:

from traitlets import Dict

class LatexPreprocessor(Preprocessor):
    # ... existing code ...

    # The single "Gateway" for all future customizations
    latex_params = Dict(
        default_value={},
        help="Dictionary of parameters to pass to the LaTeX template."
    ).tag(config=True)

    def preprocess(self, nb, resources):
        # ... existing code ...
        
        # Ensure the 'latex' resource dict exists
        if 'latex' not in resources:
            resources['latex'] = {}
        
        # Inject the entire dictionary into resources
        # Users can now pass {'toc': True, 'secnumdepth': 0, 'my_custom_flag': 'xyz'}
        resources['latex'].update(self.latex_params)
        
        return nb, resources

Edit current base.tex.j2 to include toc and secnumdepth

...
((* block docclass *))
\documentclass{article}

% ... existing setup ...

% 1. Section Numbering Depth
% Allows users to pass {'secnumdepth': 0} to remove numbering
((* if resources.latex and resources.latex.get('secnumdepth') is not none *))
    \setcounter{secnumdepth}{((( resources.latex.secnumdepth )))}
((* endif *))

% 2. Table of Contents Depth (Optional extra win)
((* if resources.latex and resources.latex.get('tocdepth') is not none *))
    \setcounter{tocdepth}{((( resources.latex.tocdepth )))}
((* endif *))

((* endblock docclass *))
...

...
((* block maketitle *))
\maketitle

% 3. Optional Table of Contents
((* if resources.latex and resources.latex.get('toc', False) *))
    \tableofcontents
    \newpage
((* endif *))

((* endblock maketitle *))
...

Further behavior was described here:
Notbook Issue 7722