Allow --config-file option in mkdocs

[fgrbr]

Hi,

First of all, thanks a lot for this great package!

I use another package that requires the option --config-file to bet set when building the documentation using mkdocs build --config-file mkdocs.yml. Unfortunately, in this case, a value error is reported ValueError: The project root dir is ambiguous ! Please report this issue to mkdocs-gallery.

In particular, when providing the absolute path mkdocs serve --config-file /workspaces/mkdocs-gallery/mkdocs.yml in noxfile.py instead of mkdocs serve, everything works as expected when running nox -s docs within the folder /workspaces/mkdocs-gallery/. However, when changing it to mkdocs serve --config-file mkdocs.yml, I get the following error:

nox -s docs
nox > Running session docs-3.9
nox > Re-using existing virtual environment at .nox/docs-3-9.
nox > 
A `pyproject.toml` file exists. Loading it.
nox > 
About to install *docs* requirements: ['mkdocs', 'matplotlib', 'seaborn', 'statsmodels', 'plotly', 'pillow'].
 Conda pkgs are ()
nox > [docs] Installing requirements with pip: ['mkdocs', 'matplotlib', 'seaborn', 'statsmodels', 'plotly', 'pillow']
nox > python -m pip install mkdocs matplotlib seaborn statsmodels plotly pillow
nox > python -m pip install .
nox > mkdocs serve --config-file mkdocs.yml
INFO     -  Building documentation...
WARNING  -  Please install 'memory_profiler' to enable peak memory measurements.
WARNING  -  optipng binaries not found, PNG images and thumbnails will not be optimized
INFO     -  generating gallery...
Traceback (most recent call last):
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/mkdocs/__main__.py", line 234, in serve_command
    serve.serve(dev_addr=dev_addr, livereload=livereload, watch=watch, **kwargs)
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/mkdocs/commands/serve.py", line 83, in serve
    builder(config)
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/mkdocs/commands/serve.py", line 76, in builder
    build(config, live_server=live_server, dirty=dirty)
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/mkdocs/commands/build.py", line 273, in build
    config.plugins.run_event('pre_build', config=config)
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/mkdocs/plugins.py", line 522, in run_event
    result = method(**kwargs)
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/mkdocs_gallery/plugin.py", line 263, in on_pre_build
    galleries_tocs, self.md_to_src = generate_gallery_md(self.config, config)
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/mkdocs_gallery/gen_gallery.py", line 418, in generate_gallery_md
    all_info = AllInformation.from_cfg(gallery_conf, mkdocs_conf)
  File "/workspaces/mkdocs-gallery/.nox/docs-3-9/lib/python3.9/site-packages/mkdocs_gallery/gen_data_model.py", line 905, in from_cfg
    raise ValueError("The project root dir is ambiguous ! Please report this issue to mkdocs-gallery.")
ValueError: The project root dir is ambiguous ! Please report this issue to mkdocs-gallery.
nox > Command mkdocs serve --config-file mkdocs.yml failed with exit code 1
nox > Session docs-3.9 failed.

I think the main problem is Path(mkdocs_conf['config_file_path']).parent in gen_data_model.py to get the project root, which is . if mkdocs serve --config-file mkdocs.yml was executed. The same is true for plugin.py.

I've created #64 to circumvent this path issue.