Specifically handle apidoc generated files

[chrisjsewell]

files generated with sphinx-autodoc will inherently contain toctree directives,
and so sphinx_external_toc will warn e.g.: apidoc/my_package.rst:7: WARNING: toctree directive not expected with external-toc [etoc.toctree]

You can currently work around this by including:

suppress_warnings = ["etoc.toctree"]

but that will then ignore any legitimate issues

so it would nice to be able to "mark" a file as "it is ok for this file and its children to contain toctree"

[mikofski]

FYI: this issue "toctree directive not expected with external-toc" also pops up using sphinx-tags